kconfig/merge_config.sh: Accept a single file

merge_config.sh can usefully be applied to a single file.
It implicitly merges with the default configuration.
Signed-off-by: Gabriel de Perthuis <g2p.code@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.com>

scripts/kconfig/merge_config.sh

@@ -77,7 +77,7 @@ while true; do
 	esac
 done
 
-if [ "$#" -lt 2 ] ; then
+if [ "$#" -lt 1 ] ; then
 	usage
 	exit
 fi