setup-disk: tell user that MBR might need be set manully

fixes #715

setup-disk.in

@@ -869,7 +869,8 @@ shift $(( $OPTIND - 1))
 if [ -d "$1" ]; then
 	# install to given mounted root
 	apk add -q syslinux
-	install_mounted_root "${1%/}"
+	install_mounted_root "${1%/}" \
+		&& echo "You might need fix the MBR to be able to boot" >&2
 	exit $?
 fi