Revert "inline kernel dtb build"

This reverts commit 8609a0fb.
This is specific to a device (or vendor) and is confusing people
building for other 8974s. A more generic implementation is required,
and until then device-local mkbootimg overloads can/should be
used.

Change-Id: Ia582f9ca73d454da4ada894b489aea883d6f5186