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)
diff --git a/payloads/libpayload/arch/x86/Makefile.inc b/payloads/libpayload/arch/x86/Makefile.inc
index 549a630..d9cc19e 100644
--- a/payloads/libpayload/arch/x86/Makefile.inc
+++ b/payloads/libpayload/arch/x86/Makefile.inc
@@ -31,6 +31,7 @@
 libc-y += main.c sysinfo.c
 libc-y += timer.c coreboot.c util.S
 libc-y += exec.S virtual.c
+libc-y += selfboot.c
 libc-y += string.c
 libc-y += exception_asm.S exception.c