Rework `TARGET_PLATFORM` into `TARGET_PLATFORM_<platform>`

**MAINTAINERS** As usual, please be sure packages build before merging.  Also, please check issue #245 for C4 API breakage violations.

Following issue #245, remove the `TARGET_PLATFORM` Makefile variable
and replace with e.g. `TARGET_PLATFORM_BEAGLEBONE`, and enable
building multiple platforms' artifacts from applicable architectures.

For users, this change is visible: `--with-platform=beaglebone` is
replaced by `--with-platform-beaglebone`.

The `./configure` logic works as follows, similar to thread flavor
selection:

- By default, build all platforms applicable to the `$host`
  architecture

- If any `--with-platform-*` specified, only build for that platform
  if applicable to `$host` architecture

- Otherwise, if any `--without-platform-*` specified, build everything
  (applicable to `$host` architecture) *except* that platform

The rest of the build system is modified to build the Beaglebone PRU
drivers and Raspberry Pi GPIO and SPI drivers depending on the above
configuration.

Debian packaging is fixed to include
`/usr/lib/linuxcnc/xenomai/pru_generic.*`.

Fixes #245
Ha!  The Buildbot had never specified `--with-platform=beaglebone`, so previously, despite the ARM build, `$TARGET_PLATFORM` defaulted to `PC` [(configure log: search for `usermode`)][2].  The `hm2-idrom` unit tests had been succeeding all along [(an example)][1].

Now that ARM architecture is automatically configuring itself, and correctly, the `hm2-idrom` drivers aren't being built [(configure log)][4]; however, the `hostmot2` unit test fails because the module isn't built [(unit test log: search for `dlopen`)][3], but the unit test runs anyway.

@cdsteinkuehler, you wrote something about wanting an ARM board with PCI running a Mesa card.  What's the right fix here?  Should the unit test be disabled when `with_usermode_pci=no`, as this code intends, or should the `hostmot2.so` module be built even for platforms other than `$TARGET_PLATFORM_PC`?

[1]: http://buildbot.dovetail-automata.com/builders/d7-bb-x-tst/builds/73
[2]: http://buildbot.dovetail-automata.com/builders/d7-bb-bld/builds/127/steps/chroot-configure/logs/stdio
[3]: http://buildbot.dovetail-automata.com/builders/d7-bb-pos-tst/builds/73/steps/runtests/logs/stdio
[4]: http://buildbot.dovetail-automata.com/builders/d7-bb-bld/builds/128/steps/chroot-configure/logs/stdio