Add platform hooks for boot redundancy support

These hooks are intended to allow one platform to try load
images from alternative places. There is a hook to initialize
the sequence of boot locations and a hook to pass to the next
sequence.

Change-Id: Ia0f84c415208dc4fa4f9d060d58476db23efa5b2
Signed-off-by: Roberto Vargas <roberto.vargas@arm.com>
diff --git a/bl2/bl2_main.c b/bl2/bl2_main.c
index 0fe82d9..018deb3 100644
--- a/bl2/bl2_main.c
+++ b/bl2/bl2_main.c
@@ -34,6 +34,9 @@
 	auth_mod_init();
 #endif /* TRUSTED_BOARD_BOOT */
 
+	/* initialize boot source */
+	bl2_plat_preload_setup();
+
 	/* Load the subsequent bootloader images. */
 	next_bl_ep_info = bl2_load_images();