feat(fvp): enable external SP images in BL2 config

Currently the list of SP UUIDs loaded by BL2 is hardcoded in the DT.
This is a problem when building a system with other SPs (e.g. from
Trusted Services). This commit implements a workaround to enable adding
SP UUIDs to the list at build time.

Signed-off-by: Balint Dobszay <balint.dobszay@arm.com>
Change-Id: Iff85d3778596d23d777dec458f131bd7a8647031
diff --git a/Makefile b/Makefile
index b4bebf1..8411355 100644
--- a/Makefile
+++ b/Makefile
@@ -525,6 +525,10 @@
         ifeq ($(TS_SP_FW_CONFIG),1)
             DTC_CPPFLAGS	+=	-DTS_SP_FW_CONFIG
         endif
+
+        ifneq ($(ARM_BL2_SP_LIST_DTS),)
+            DTC_CPPFLAGS += -DARM_BL2_SP_LIST_DTS=$(ARM_BL2_SP_LIST_DTS)
+        endif
     else
         # All other SPDs in spd directory
         SPD_DIR := spd