kernel: add CONFIG_MDEF

Easier to build logic for when an MDEF file is to be parsed since
unified kernel needs to do it as well. Can also be useful for testing,
when toggling between static and dynamic objects in the same test case.

Change-Id: I51eb8919e18443516ade13caab04698d37d91803
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>