Makefile: add the xconfig target to the help of the main Makefile

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Makefile

@@ -561,6 +561,7 @@ help:
 	@echo
 	@echo 'Configuration:'
 	@echo '  menuconfig             - interactive curses-based configurator'
+	@echo '  xconfig		- interactive Qt-based configurator'
 	@echo '  oldconfig              - resolve any unresolved symbols in .config'
 	@echo '  configured             - make {uclibc/busybox/linux26}-config'
 	@echo '  saveconfig             - save current configuration under local/<project>'