omap4xxx: add flag to allow custom domx

Make it possible for a device to provide its own TI domx
by setting the BOARD_USE_TI_CUSTOM_DOMX flag and including
the domx-headers with TARGET_SPECIFIC_HEADER_PATH.

This replaces "[1/3] omap4xxx: Allow custom DOMX source instead of
Google DOMX default" from cm-10.{1,2} which made it necessary to
also patch frameworks/{av,base}. This is no longer needed when
using TARGET_SPECIFIC_HEADER_PATH to inject the domx-headers.

Change-Id: I09b116fb722ec7f77c5a4b1b4b4138bdd11c6dd6