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/bl32/sp_min/sp_min.mk b/bl32/sp_min/sp_min.mk
index 6339cf8..590b032 100644
--- a/bl32/sp_min/sp_min.mk
+++ b/bl32/sp_min/sp_min.mk
@@ -1,5 +1,5 @@
 #
-# Copyright (c) 2016-2020, ARM Limited and Contributors. All rights reserved.
+# Copyright (c) 2016-2021, ARM Limited and Contributors. All rights reserved.
 #
 # SPDX-License-Identifier: BSD-3-Clause
 #
@@ -8,6 +8,7 @@
 	$(error SP_MIN is only supported on AArch32 platforms)
 endif
 
+include lib/extensions/amu/amu.mk
 include lib/psci/psci_lib.mk
 
 INCLUDES		+=	-Iinclude/bl32/sp_min
@@ -27,9 +28,8 @@
 BL32_SOURCES		+=	lib/pmf/pmf_main.c
 endif
 
-ifeq (${ENABLE_AMU}, 1)
-BL32_SOURCES		+=	lib/extensions/amu/aarch32/amu.c\
-				lib/extensions/amu/aarch32/amu_helpers.S
+ifeq (${ENABLE_AMU},1)
+BL32_SOURCES		+=	${AMU_SOURCES}
 endif
 
 ifeq (${WORKAROUND_CVE_2017_5715},1)