This change introduces the `amu.mk` Makefile, used to remove the need
to manually include AMU sources into the various build images.
Makefiles requiring the list of AMU sources are expected to include
this file and use `${AMU_SOURCES}` to retrieve them.
Change-Id: I3d174033ecdce6439a110d776f0c064c67abcfe0
Signed-off-by: Chris Kay <chris.kay@arm.com>
Chris Kay