mips: do not place branch instructions in branch delay slot

A branch instruction in a branch delay slot confuses the execution
pipeline and causes an exception.

bootblock.S was written 'by hand', has a branch instruction in branch
delay slot and includes '.set noreorder' directive, which causes it to
crash when trying to branch to main().

Adding a nop instruction fixes the problem. Also adding a nop after
the last branch in the file just in case main() returns and the object
linked next starts with a branch.

BUG=chrome-os-partner:31438
TEST=Running on the simulator can reach main() now

Change-Id: I0882b2eb5ce426f5a311018ffbb6f37a2ca64d98
Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/221421
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/9183
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
Tested-by: build bot (Jenkins)

src/arch/mips/bootblock.S

@@ -39,6 +39,10 @@ _start:
 	/* Run main */
 	b	main
 
-	/* Should never return from main. */
-2:
+	/*
+	 * Should never return from main. Make sure there is no branch in the
+	 * branch delay slot.
+	 */
+2:	nop
 	b	2b
+	nop	/* Make sure there is no branch after this either. */