generate_medkit: change default output name

This changes output name to format that is consistent with one used in
3.x release cycle for Turris Omnia.

generate_medkit

@@ -124,13 +124,13 @@ done
 [ -n "$OUTPUT" ] || {
 	case "$BOARD" in
 		mox)
-			OUTPUT=medkit-mox.tar.gz
+			OUTPUT=mox-medkit.tar.gz
 			;;
 		omnia)
-			OUTPUT=medkit-omnia.tar.gz
+			OUTPUT=omnia-medkit.tar.gz
 			;;
 		turris)
-			OUTPUT=medkit-turris.tar.gz
+			OUTPUT=turris-medkit.tar.gz
 			;;
 	esac
 }