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>

payloads/Kconfig

@@ -59,5 +59,12 @@ config PAYLOAD_IS_FLAT_BINARY
 	  Add the payload to cbfs as a flat binary type instead of as an
 	  elf payload
 
-endmenu
+config COREINFO_SECONDARY_PAYLOAD
+	bool "Load coreinfo as a secondary payload"
+	default n
+	depends on ARCH_X86
+	help
+	  coreinfo can be loaded as a secondary payload under SeaBIOS, GRUB,
+	  or any other payload that can load additional payloads.
 
+endmenu