mkconfig: start deprecating Makefile config targets

Now that we've got boards.cfg and most people have converted over,
start warning people who have yet to so we can phase board configs
completely out of the Makefile.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/mkconfig b/mkconfig
index ecb6d4e..438530b 100755
--- a/mkconfig
+++ b/mkconfig
@@ -29,6 +29,15 @@
 	set ${line}
 	# add default board name if needed
 	[ $# = 3 ] && set ${line} ${1}
+elif [ "${MAKEFLAGS+set}${MAKELEVEL+set}" = "setset" ] ; then
+	# only warn when using a config target in the Makefile
+	cat <<-EOF
+
+	warning: Please migrate to boards.cfg.  Failure to do so will
+	         mean removal of your board in the next release.
+
+	EOF
+	sleep 5
 fi
 
 while [ $# -gt 0 ] ; do