refactor(cm): introduce INIT_UNUSED_NS_EL2 macro

Introducing INIT_UNUSED_NS_EL2 macro which guards the code that
disables the unused EL2 when a platform hands off from EL3
to NS-EL1 instead of NS-EL2. Platforms without NS-EL2 in use
must enable this flag.

BREAKING CHANGE: Initialisation code for handoff from EL3 to NS-EL1
disabled by default. Platforms which do that need to enable this macro
going forward

Signed-off-by: Arvind Ram Prakash <arvind.ramprakash@arm.com>
Change-Id: I61431cc4f7e2feb568d472828e5fd79cc73e51f5
diff --git a/make_helpers/defaults.mk b/make_helpers/defaults.mk
index e02917c..f0f157c 100644
--- a/make_helpers/defaults.mk
+++ b/make_helpers/defaults.mk
@@ -367,3 +367,9 @@
 # Disabled by default because it constitutes an attack vector into TF-A. It
 # should only be enabled if there is a use case for it.
 ENABLE_CONSOLE_GETC		:= 0
+
+# Build option to disable EL2 when it is not used.
+# Most platforms switch from EL3 to NS-EL2 and hence the unused NS-EL2
+# functions must be enabled by platforms if they require it.
+# Disabled by default.
+INIT_UNUSED_NS_EL2		:= 0