libpayload: arm: add code to clear bss

This adds some assembly code to clear .bss segment. It might have been
already cleared by the loader, but it is not guaranteed. This also
helps when the program is loaded by the debugger.

BUG=none
TEST=observed that .bss is now initialized when the program is
     restarted. Verified correct boundaries of the segment.

Change-Id: I0aed0070da53881e4cf8c27049459040c006e765
Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/201784
Reviewed-by: Vincent Palatin <vpalatin@chromium.org>
Reviewed-by: Trevor Bourget <tbourget@codeaurora.org>

payloads/libpayload/arch/arm/head.S

@@ -34,14 +34,25 @@
  */
 ENTRY(_entry)
 
+	/* TODO: disable interrupts */
+
+	/* Clear BSS */
+	mov r12, #0
+	ldr r2, bss_boundaries
+	ldr r3, bss_boundaries + 4
+	cmp r2, r3
+	beq no_bss
+
+	sub r3, #4 /* Account for late condition check. */
+loop:
+	cmp r2, r3
+	str r12, [r2], #4
+	blt loop
+no_bss:
 	/* Save off the location of the coreboot tables */
 	ldr r1, 1f
 	str r0, [r1]
 
-	/* TODO: disable interrupts */
-
-	/* TODO: Clear BSS */
-
 	/* Setup new stack */
 	ldr sp, 2f
 
@@ -59,6 +70,9 @@ ENTRY(_entry)
 ENDPROC(_entry)
 
 .align 4
+bss_boundaries:
+	.word _edata
+	.word _end
 1:
 .word	cb_header_ptr
 2: