build: add -I$(srctree)/kernel/unified/include to path

This will allow making some arch-specfic parts arch-agnostic. Some
definitions that are currently in arch/<arch>/include will end up in
kernel/unified/include.

Change-Id: I4df71090e20a5599e70fb578b36f9211411b56ad
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>