Introduce RECLAIM_INIT_CODE build flag

This patch introduces a build flag "RECLAIM_INIT_CODE" to mark boot time
code which allows platforms to place this memory in an appropriate
section to be reclaimed later. This features is primarily targeted for
BL31. Appropriate documentation updates are also done.

Change-Id: If0ca062851614805d769c332c771083d46599194
Signed-off-by: Daniel Boulby <daniel.boulby@arm.com>
diff --git a/make_helpers/defaults.mk b/make_helpers/defaults.mk
index 7df4cd2..435de20 100644
--- a/make_helpers/defaults.mk
+++ b/make_helpers/defaults.mk
@@ -146,6 +146,10 @@
 # platform Makefile is free to override this value.
 SEPARATE_CODE_AND_RODATA	:= 0
 
+# If the BL31 image initialisation code is recalimed after use for the secondary
+# cores stack
+RECLAIM_INIT_CODE		:= 0
+
 # Default to SMCCC Version 1.X
 SMCCC_MAJOR_VERSION		:= 1