kbuild: automatically include southbridges

This change switches all southbridge vendors and southbridges
to be autoincluded by Makefile.inc, rather than having to be
mentioned explicitly in southbridge/Makefile.inc or in
southbridge/<vendor>/Makefile.inc.

In order to be able to drop southbridge/amd/Makefile.inc, some
scattered source files had to be moved to a southbridge/amd/common
directory, in accordance to what we are doing on other architectures
already.

This means, vendor and southbridge directories are now "drop
in", e.g. be placed in the coreboot directory hierarchy
without having to modify any higher level coreboot files.

The long term plan is to enable out of tree components to be
built with a given coreboot version (given that the API did not
change).

Change-Id: I79bd644a0a3c4e8320c80f8cc7a7f8ffd65d32f2
Signed-off-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-on: http://review.coreboot.org/9796
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

Makefile.inc

@@ -52,11 +52,10 @@ PHONY+= clean-abuild coreboot lint lint-stable build-dirs
 
 #######################################################################
 # root source directories of coreboot
-subdirs-y := src/lib src/console src/device src/ec src/southbridge src/soc
+subdirs-y := src/lib src/console src/device src/ec $(wildcard src/southbridge/*/*) src/soc
 subdirs-y += src/northbridge src/superio src/drivers src/cpu src/vendorcode
 subdirs-y += util/cbfstool util/sconfig util/nvramtool util/broadcom
-subdirs-y += src/arch/arm src/arch/arm64 src/arch/mips src/arch/riscv
-subdirs-y += src/arch/x86
+subdirs-y += $(wildcard src/arch/*)
 subdirs-y += src/mainboard/$(MAINBOARDDIR)
 
 subdirs-y += site-local