Rewrite multiarch task and design information.

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+[[!meta title="Multiarch Design"]]
+
+There are three high-level design aspects to this multiarch implementation: the
+filesystem hierarchy that primarily enables package coinstallability, control
+information documenting package coinstallability and dependency satisfaction,
+and coinstallability considerations in package management.
+
+For reference, see the [specification][wuc-mas] and [other
+documentation][wdo-ma] for multiarch in Debian and Ubuntu.
+
+
+Filesystem Hierarchy
+====================
+
+For packages to be coninstallable with themselves (such that a user can have
+multiple architecture builds of one package installed simultaneously), all
+architecture-dependent files must be installed in architecture-dependent
+locations.  The most straightforward way to do this is to embed architecture
+strings into filesystem paths.
+
+Debian and Ubuntu do something similar to this.  In a Debian library package
+that supports multiarch, shared object files are installed in
+`/usr/lib/<triplet>`, where `<triplet>` is a [GNU system type][ac-systemtype].
+
+The scope of Debian's and Ubuntu's multiarch work [does not include][wdo-ma-fut]
+coinstallable executable programs.
+
+Such work, however, would help enable cross installation of packages.  So a goal
+of this multiarch implementation is to support coinstallation of shared object
+files, header files, and executable programs.
+
+There are two proposed filesystem hierarchies (see below) to support this.
+
+Parts of the toolchain (especially the dynamic linker) need to be configured
+and/or patched to use multiarch library paths.  Additionally, at least the
+native-architecture executable program directories have to be added to the PATH
+environment variable.
+
+Proposal 1: `/usr` Organized Primarily by Architecture
+------------------------------------------------------
+
+It may be useful to install all architecture-dependent files under
+`/usr/<arch>`, where `<arch>` is a distribution architecture string.  The
+filesystem hierarchy would then look something like this:
+
+    /
+     +- bin/
+     |   +- core-linux-eglibc/
+     |   \- cortexa8-linux-eglibc/
+     +- lib/
+     |   +- core-linux-eglibc/
+     |   \- cortexa8-linux-eglibc/
+     +- sbin/
+     |   +- core-linux-eglibc/
+     |   \- cortexa8-linux-eglibc/
+     \- usr/
+         +- core-linux-eglibc/
+         |   +- bin/
+         |   +- games/
+         |   +- include/
+         |   +- lib/
+         |   +- sbin/
+         +- cortexa8-linux-eglibc/
+         |   +- bin/
+         |   +- games/
+         |   +- include/
+         |   +- lib/
+         |   +- sbin/
+         +- bin/
+         +- games/
+         +- include/
+         +- local/
+         +- sbin/
+         +- share/
+         \- src/
+
+Note that `/usr/lib` doesn't exist, as no architecture-independent files should
+be installed there.
+
+Proposal 2: `/usr` Organized Secondarily by Architecture
+--------------------------------------------------------
+
+It may be cleaner and slightly more efficient to install architecture-dependent
+files within an architecture-dependent directory under otherwise standard paths.
+This is similar to how Debian and Ubuntu have designed their multiarch
+filesystem hierarchy.  Such an organized filesystem hierarchy would look
+something like this:
+
+    /
+     +- bin/
+     |   +- core-linux-eglibc/
+     |   \- cortexa8-linux-eglibc/
+     +- lib/
+     |   +- core-linux-eglibc/
+     |   \- cortexa8-linux-eglibc/
+     +- sbin/
+     |   +- core-linux-eglibc/
+     |   \- cortexa8-linux-eglibc/
+     \- usr/
+         +- bin/
+             +- core-linux-eglibc/
+             +- cortexa8-linux-eglibc/
+         +- games/
+             +- core-linux-eglibc/
+             +- cortexa8-linux-eglibc/
+         +- include/
+             +- core-linux-eglibc/
+             +- cortexa8-linux-eglibc/
+         +- lib/
+             +- core-linux-eglibc/
+             +- cortexa8-linux-eglibc/
+         +- local/
+         +- sbin/
+             +- core-linux-eglibc/
+             +- cortexa8-linux-eglibc/
+         +- share/
+         \- src/
+
+
+Control Information
+===================
+
+New package control information will be needed to indicate whether a package is
+coinstallable with itself and to mark its purpose.
+
+During build-time, there are two possible architectures for any dependent package:
+
+  * Host-architecture (architecture for which packages are built).  Packages for
+    this architecture provide things like libraries and headers.  An example is
+    libexpat.1-dev.
+  * Build-architecture (architecture on which packages are built).  Packages for
+    this architecture provide things like build utilities An example is
+    pkg-config.
+
+During install-time, there are also two possible architectures for any dependent
+package:
+
+  * Host-architecture (architecture for which packages are installed).  Packages
+    for this architecture provide things like libraries and utilities.  Examples
+    include libz.1, gcc-4.7-<arch> (dependency of gcc-<arch>), and fakeroot
+    (recommendation of opkhelper-1.0).
+  * Install-architecture (architecture on which packages are installed).
+    Packages for this architecture provide things like utilities used by
+    maintainer scripts.  An example is insserv (or a similar tool).
+
+Host-architecture packages should be coinstallable with themselves, and they
+should satisfy dependencies of the same architecture.  For example, the
+dependency of libexpat.1-dev:cortexa8-linux-eglibc on libexpat.1 should resolve
+to a dependency on libexpat.1:cortexa8-linux-eglibc.
+
+Build- and install-architecture packages need not be coinstallable with
+themselves, and they should satisfy dependencies of any architecture.  For
+example, the build dependency of glib (built on core-linux-eglibc for
+cortexa8-linux-eglibc) on pkg-config should resolve to a build dependency on
+pkg-config:core-linux-eglibc.
+
+Debian and Ubuntu specify such properties in a `Multi-Arch` control field.  A
+value of `same` indicates that a package satisfies dependencies of the same
+architecture; a value of `foreign` indicates that a package satisfies
+dependencies of any architecture.  We can implement something similar, perhaps
+even using the same terminology.
+
+Logic to handle the new control information will need to be added to opkg and
+opkhelper (specifically oh-checkbuilddeps).
+
+Special design considerations need to be given to install-time dependencies.
+Specific use case analysis can help here.
+
+
+Architecture-Independent Files
+==============================
+
+Many packages provide architecture-independent files (configuration files,
+documentation, data, etc.).  To the extent possible and feasible, these should
+simply be provided by architecture-independent packages (with names like
+"*-common", "*-data", or "*-base").
+
+If this is not possible in all cases, then some modifications to opkg will be
+necessary (reference counting, implicit "Breaks" relationships, etc.).
+
+
+[wdo-ma]: http://wiki.debian.org/Multiarch
+[wuc-mas]: https://wiki.ubuntu.com/MultiarchSpec
+[ac-systemtype]: https://www.gnu.org/savannah-checkouts/gnu/autoconf/manual/autoconf-2.69/html_node/System-Type.html
+[wdo-ma-fut]: http://wiki.debian.org/Multiarch#Future_development