feat(handoff): introduce firmware handoff library

Add transfer list APIs and firmware handoff build option.

Change-Id: I68a0ace22c7e50fcdacd101eb76b271d7b76d8ff
Signed-off-by: Raymond Mao <raymond.mao@linaro.org>
diff --git a/make_helpers/defaults.mk b/make_helpers/defaults.mk
index aaabb27..b7e6f99 100644
--- a/make_helpers/defaults.mk
+++ b/make_helpers/defaults.mk
@@ -147,6 +147,9 @@
 # by lower ELs.
 HANDLE_EA_EL3_FIRST_NS		:= 0
 
+# Enable Handoff protocol using transfer lists
+TRANSFER_LIST			:= 0
+
 # Secure hash algorithm flag, accepts 3 values: sha256, sha384 and sha512.
 # The default value is sha256.
 HASH_ALG			:= sha256