Introduce PROGRAMMABLE_RESET_ADDRESS build option

This patch introduces a new platform build option, called
PROGRAMMABLE_RESET_ADDRESS, which tells whether the platform has
a programmable or fixed reset vector address.

If the reset vector address is fixed then the code relies on the
platform_get_entrypoint() mailbox mechanism to figure out where
it is supposed to jump. On the other hand, if it is programmable
then it is assumed that the platform code will program directly
the right address into the RVBAR register (instead of using the
mailbox redirection) so the mailbox is ignored in this case.

Change-Id: If59c3b11fb1f692976e1d8b96c7e2da0ebfba308
diff --git a/bl1/aarch64/bl1_entrypoint.S b/bl1/aarch64/bl1_entrypoint.S
index 147c930..4fc5291 100644
--- a/bl1/aarch64/bl1_entrypoint.S
+++ b/bl1/aarch64/bl1_entrypoint.S
@@ -42,9 +42,15 @@
 	 */
 
 func bl1_entrypoint
+	/* ---------------------------------------------------------------------
+	 * If the reset address is programmable then bl1_entrypoint() is
+	 * executed only on the cold boot path. Therefore, we can skip the warm
+	 * boot mailbox mechanism.
+	 * ---------------------------------------------------------------------
+	 */
 	el3_entrypoint_common					\
 		_set_endian=1					\
-		_warm_boot_mailbox=1				\
+		_warm_boot_mailbox=!PROGRAMMABLE_RESET_ADDRESS	\
 		_secondary_cold_boot=1				\
 		_init_memory=1					\
 		_init_c_runtime=1				\