libpayload/Makefile.payload: Include libpayload's .config

It's required to tell xcompile what compiler to use.

Change-Id: I9f1ddef96a20df1d83bfd4883b2e006ba78ce7c4
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/62269
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Thomas Heijligen <src@posteo.de>

payloads/libpayload/Makefile.payload

@@ -56,6 +56,8 @@ export V
 
 ifeq ($(filter %clean,$(MAKECMDGOALS)),)
 
+-include $(LIBPAYLOAD_DOTCONFIG)
+
 xcompile	:= $(obj)/xcompile
 xcompile_script	:= $(LIBPAYLOAD_SRC)/../../util/xcompile/xcompile