arm: allow custom stage entry code

this change defines stage_entry as a weak symbol so that a board
can implement custom stage entry code.

BUG=none
BRANCH=tot
TEST=built all current boards. booted cosmos p1.

Change-Id: If8f6945ecdc5047558bb6359aa997867e36f33b9
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Original-Commit-Id: 86d5008981d0b01652907baab47a476d784a2ceb
Original-Change-Id: Ib43158c4013e6393d86a9aef37cf444a48b9fc79
Original-Signed-off-by: Daisuke Nojiri <dnojiri@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/238021
Original-Reviewed-by: Julius Werner <jwerner@chromium.org>
Original-Reviewed-by: Furquan Shaikh <furquan@chromium.org>
Reviewed-on: http://review.coreboot.org/9721
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/arch/arm/stages.c

@@ -31,7 +31,10 @@
 #include <arch/stages.h>
 #include <arch/cache.h>
 
-void stage_entry(void)
+/**
+ * generic stage entry point. override this if board specific code is needed.
+ */
+__attribute__((weak)) void stage_entry(void)
 {
 	main();
 }