build_system: Extend site-local

- Add a target at the end of the build that can be used to run additional
scripts or additional targets after coreboot.rom is built.
- Source a site-local Kconfig file to allow site-specific configuration.

This eliminates the need to add a hook for a script at the end of the
build because you can add one yourself in site-local.

Example site-local/Makefile.inc:

build_complete::
ifeq ($(CONFIG_SITE_LOCAL),y)
	echo "Running additional steps in site-local"
	# run some script here to make my build unreproducible.
endif

.phony: build_complete

Example site-local/Kconfig:

menu "site-local"

config SITE_LOCAL
	bool "site-local enabled"
	help
	  Enable my site-local configuration to do stuff.

endmenu

Change-Id: Id4d1e727c69b5cdb05e7d52731bbb1d1e201864a
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/13413
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: build bot (Jenkins)

src/Kconfig

@@ -407,6 +407,9 @@ config FMDFILE
 
 endmenu
 
+# load site-local kconfig to allow user specific defaults and overrides
+source "site-local/Kconfig"
+
 config SYSTEM_TYPE_LAPTOP
 	default n
 	bool