libpayload: Move over to the payload's stack during startup

Don't keep using the coreboot stack on ARMv7.

Change-Id: I734c5d77f8584e30ee0c720d41e21e3040f56db4
Signed-off-by: Gabe Black <gabeblack@google.com>
Reviewed-on: http://review.coreboot.org/2668
Tested-by: build bot (Jenkins)
Reviewed-by: Marc Jones <marc.jones@se-eng.com>
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

payloads/libpayload/arch/armv7/head.S

@@ -31,14 +31,21 @@
 	.text
 	.align 4
 
+1:
+.word	_stack
+
 /*
  * Our entry point
  */
 _entry:
 
 	/* TODO: disable interrupts */
+
 	/* TODO: Clear BSS */
-	/* TODO: Setup new stack */
+
+	/* Setup new stack */
+	ldr sp, 1b
+
 	/* TODO: Save old stack pointer */
 
 	/* Let's rock. */