build: toplevel.mk: fix missing kconfig dependency tracking

Toplevel Make is not aware about changes in the `scripts/config/*conf`
targets and this is causing issues for during update to that part of
build tree, where one needs to handle this manually by either force
rebuilding the targets or running `make config-clean`. Fix this by
forcing the rebuild if necessary.

Fixes: openwrt#9297
Signed-off-by: Petr Štetiar <ynezz@true.cz>

include/toplevel.mk

@@ -101,7 +101,7 @@ ifneq ($(DISTRO_PKG_CONFIG),)
 scripts/config/%onf: export PATH:=$(dir $(DISTRO_PKG_CONFIG)):$(PATH)
 endif
 scripts/config/%onf: CFLAGS+= -O2
-scripts/config/%onf:
+scripts/config/%onf: FORCE
 	@$(_SINGLE)$(SUBMAKE) $(if $(findstring s,$(OPENWRT_VERBOSE)),,-s) \
 		-C scripts/config $(notdir $@)