bl2-el3: Add BL2_EL3 image

This patch enables BL2 to execute at the highest exception level
without any dependancy on TF BL1. This enables platforms which already
have a non-TF Boot ROM to directly load and execute BL2 and subsequent BL
stages without need for BL1.  This is not currently possible because
BL2 executes at S-EL1 and cannot jump straight to EL3.

Change-Id: Ief1efca4598560b1b8c8e61fbe26d1f44e929d69
Signed-off-by: Roberto Vargas <roberto.vargas@arm.com>
diff --git a/make_helpers/defaults.mk b/make_helpers/defaults.mk
index fa0d17d..643890f 100644
--- a/make_helpers/defaults.mk
+++ b/make_helpers/defaults.mk
@@ -27,6 +27,9 @@
 # Base commit to perform code check on
 BASE_COMMIT			:= origin/master
 
+# Execute BL2 at EL3
+BL2_AT_EL3			:= 0
+
 # By default, consider that the platform may release several CPUs out of reset.
 # The platform Makefile is free to override this value.
 COLD_BOOT_SINGLE_CPU		:= 0