Rework `TARGET_PLATFORM` into `TARGET_PLATFORM_<platform>`

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
Skip `hm2-idrom` test when not applicable

When running userland threads and `hostmot2.so` is not built, skip the
test.

Beaglebone/ARM architecture disables `USER_PCI`, and the `hostmot2.so`
driver therefore isn't built.
Merge pull request #257 from zultron/arm-bb-pru

Rework `TARGET_PLATFORM` into `TARGET_PLATFORM_<platform>`