libpayload: Drop duplicate copy of Kconfig

It's perfectly fine to have one single copy of kconfig in the tree.

Change-Id: Icfe32f0249dfc1c223009d6e7136462f8f8a7248
Signed-off-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-on: http://review.coreboot.org/10521
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

util/kconfig/Makefile

@@ -16,7 +16,7 @@ ifeq ($(_OS),MINGW32)
 	regex-objs=regex.o
 endif
 
-Kconfig := src/Kconfig
+Kconfig ?= src/Kconfig
 
 xconfig: $(objk)/qconf
 	$< $(Kconfig)