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

	@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>'