brillo: Add helper to conditionally include hal makefiles

This allows peripherals to define a hal.mk alongside their
peripheral.mk. The hal.mk will be included as an Android.mk
would, if the peripheral.mk is selected by the device definition

BUG: 26472263

Change-Id: Id66679549c1bc679e3a003000bba9bea95a3c39a
2 files changed