payloads: Reorganize Makefile.incs for external payloads

This is not going as far as I would like it to go, but
some of the external payloads have to be fixed up first.
Long term, I would like to  directly add payloads/external/*
to subdirs-y and remove one layer of indirection from the
build process.

For now, moving the payload Makefile targets into payloads/
is already a small improvement.

Change-Id: Ie4eb492eb804e0aaaf1a4d90af2f876f27a32a75
Signed-off-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-on: http://review.coreboot.org/10829
Reviewed-by: Martin Roth <gaumless@gmail.com>
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

Makefile.inc

@@ -59,6 +59,7 @@ subdirs-y += src/superio $(wildcard src/drivers/*) src/cpu src/vendorcode
 subdirs-y += util/cbfstool util/sconfig util/nvramtool util/broadcom
 subdirs-y += $(wildcard src/arch/*)
 subdirs-y += src/mainboard/$(MAINBOARDDIR)
+subdirs-y += payloads/external
 
 subdirs-y += site-local