libpayload: Keep a copy of .config in the build dir

This should make it easier to find the correct config for in-tree
builds.

Change-Id: I08d396ae3cedc65f63c4b8865701ea123c7d56cb
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/47628
Reviewed-by: Raul Rangel <rrangel@chromium.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/payloads/libpayload/Makefile.inc b/payloads/libpayload/Makefile.inc
index 6835b84..fc679dd 100644
--- a/payloads/libpayload/Makefile.inc
+++ b/payloads/libpayload/Makefile.inc
@@ -69,7 +69,10 @@
 CFLAGS += -flto
 endif
 
-$(obj)/libpayload-config.h: $(KCONFIG_AUTOHEADER)
+$(obj)/libpayload.config: $(DOTCONFIG)
+	cp $< $@
+
+$(obj)/libpayload-config.h: $(KCONFIG_AUTOHEADER) $(obj)/libpayload.config
 	cmp $@ $< 2>/dev/null || cp $< $@
 
 library-targets = $(addsuffix .a,$(addprefix $(obj)/,$(libraries))) $(obj)/libpayload.a