bcm281xx: boot0 hook: adjust to unified boot0 semantics

This updates the BCM281xx boot0-hook to the updated boot0 semantics
by emitting _start and the vector table before the boot0 hook (as
was the case before).

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>

arch/arm/include/asm/arch-bcm281xx/boot0.h

@@ -5,5 +5,7 @@
  */
 
 /* BOOT0 header information */
+_start:
+	ARM_VECTORS
 	.word	0xbabeface
 	.word	_end - _start