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
Fixed in #260.