include/toplevel.mk: Add xconfig target

Add xconfig target to include/toplevel.mk, so that ``make xconfig`` can
be invoked from $TOPDIR to use Qt based configuration tool to prepare
.config file.

The qconf related sources are taken from linux 4.9.13 archive.

Signed-off-by: Alif M. Ahmad <[email protected]>

include/toplevel.mk

@@ -104,6 +104,9 @@ scripts/config/mconf:
 
 $(eval $(call rdep,scripts/config,scripts/config/mconf))
 
+scripts/config/qconf:
+	@$(_SINGLE)$(SUBMAKE) -s -C scripts/config qconf CC="$(HOSTCC_WRAPPER)"
+
 scripts/config/conf:
 	@$(_SINGLE)$(SUBMAKE) -s -C scripts/config conf CC="$(HOSTCC_WRAPPER)"
 
@@ -136,6 +139,12 @@ menuconfig: scripts/config/mconf prepare-tmpinfo FORCE
 	[ -L .config ] && export KCONFIG_OVERWRITECONFIG=1; \
 		$< Config.in
 
+xconfig: scripts/config/qconf prepare-tmpinfo FORCE
+	if [ \! -e .config -a -e $(HOME)/.openwrt/defconfig ]; then \
+		cp $(HOME)/.openwrt/defconfig .config; \
+	fi
+	$< Config.in
+
 prepare_kernel_conf: .config FORCE
 
 ifeq ($(wildcard staging_dir/host/bin/quilt),)