libpayload: Keep stack boundary small on x86

There is no measurable performance impact, but
this positively impacts the memory used by payloads.

Change-Id: Ib2bdba4a7bf2a4c2391a20b3225bbb44422d3194
Signed-off-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-on: http://review.coreboot.org/10751
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

payloads/libpayload/arch/x86/Makefile.inc

@@ -27,6 +27,8 @@
 ## SUCH DAMAGE.
 ##
 
+CFLAGS += -mpreferred-stack-boundary=2
+
 head.o-y += head.S
 libc-y += main.c sysinfo.c
 libc-y += timer.c coreboot.c util.S