stabilize app: deprecate HIL from build system
stabilize app: deprecate HIL from build system

* the hil build has not been maintained since feb
  and would require fundamental changes to the new
  ivory/tower architecture. reinstate it if we
  ever decide to support it again.

GaloisInc/ivory-tower-stm32,GaloisInc/ivory-tower-stm32,GaloisInc/ivory-tower-stm32