Rework `TARGET_PLATFORM` into `TARGET_PLATFORM_<platform>`

**MAINTAINERS**  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
> @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`?

For now it doesn't much matter, but I expect Machinekit will be in use
on one of the many ARM platforms that include PCI/PCIe slots in the near
future.  IMHO, the hm2 PCI driver and associated unit tests should be
enabled for the ARM platform by default, with any required fixes
considered low priority (at least until someone actually starts using a
mesa card on an ARM board).

--
Charles