payloads: Load coreinfo as a secondary payload

This allows coreinfo to be added to CBFS as a 'secondary'
payload on x86 systems, to be loaded by the main payload
if desired.

Selecting this option, which defaults to no, builds the coreinfo
payload and adds it to CBFS as `img/coreinfo` which can then be
loaded by for example SeaBIOS or GRUB.

Change-Id: I52661d486823bc4bb215ce92dca118c9d2c2a309
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/13728
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

Makefile.inc

@@ -74,7 +74,7 @@ subdirs-y += util/cbfstool util/sconfig util/nvramtool util/broadcom
 subdirs-y += util/futility util/marvell
 subdirs-y += $(wildcard src/arch/*)
 subdirs-y += src/mainboard/$(MAINBOARDDIR)
-subdirs-y += payloads/external
+subdirs-y += payloads payloads/external
 
 subdirs-y += site-local