build(amu): introduce `amu.mk`

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>
diff --git a/bl31/bl31.mk b/bl31/bl31.mk
index 106d410..4e9e99f 100644
--- a/bl31/bl31.mk
+++ b/bl31/bl31.mk
@@ -22,6 +22,7 @@
   endif
 endif
 
+include lib/extensions/amu/amu.mk
 include lib/psci/psci_lib.mk
 
 BL31_SOURCES		+=	bl31/bl31_main.c				\
@@ -78,8 +79,7 @@
 endif
 
 ifeq (${ENABLE_AMU},1)
-BL31_SOURCES		+=	lib/extensions/amu/aarch64/amu.c		\
-				lib/extensions/amu/aarch64/amu_helpers.S
+BL31_SOURCES		+=	${AMU_SOURCES}
 endif
 
 ifeq (${ENABLE_SVE_FOR_NS},1)