ARM: Simplify board specification

* Don't assume verdex as it isn't clear this was
  occurring.
* Make an educated guess on HAIKU_BOOT_PLATFORM
  based on provided board (but still allow it to
  be overridden)
* Error out if user doesn't populate
  HAIKU_BOOT_PLATFORM or enters an unknown board
  name.
* You need to add "-sHAIKU_BOOT_BOARD=xxx" to
  your jam to build for the proper ARM device.
* Rename beagle to beagleboneblk as per the
  documentation.