libpayload: Provide selfboot function.

The calling convention of payload entry function is different by architecture.
For example, X86 takes no arguments and ARM needs first param to be a
cb_header_ptr*.

To help payloads load and execute other payloads easily and correctly, we should
provide the selfboot() function in libpayload, using same prototype as defined
in coreboot environment.

BUG=none
TEST=emerge-nyan libpayload # pass
BRANCH=none

Original-Change-Id: I8f1cb2c0df788794b2f6f7f5500a3910328a4f84
Original-Signed-off-by: Hung-Te Lin <hungte@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/199503
Original-Reviewed-by: Stefan Reinauer <reinauer@chromium.org>
(cherry picked from commit 1e916cf021ce68886eb9668982c392eadedc7b7e)
Signed-off-by: Marc Jones <marc.jones@se-eng.com>

Change-Id: I7279ef27f49ef581d25a455dd8f1f2f7f1ba58cb
Reviewed-on: http://review.coreboot.org/7907
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: build bot (Jenkins)

payloads/libpayload/arch/arm/Makefile.inc

@@ -38,6 +38,7 @@ libc-y += virtual.c
 libc-y += memcpy.S memset.S memmove.S
 libc-y += exception_asm.S exception.c
 libc-y += cache.c cpu.S
+libc-y += selfboot.c
 libcbfs-$(CONFIG_LP_CBFS) += dummy_media.c
 
 # Add other classes here when you put assembly files into them!