common: commands: make commands static

Since most commands are not public, make them static. This
prevents warnings that no common prototype is available.

Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>

common/cmd_ubi.c

@@ -287,7 +287,7 @@ out_err:
 	return err;
 }
 
-int ubi_volume_continue_write(char *volume, void *buf, size_t size)
+static int ubi_volume_continue_write(char *volume, void *buf, size_t size)
 {
 	int err = 1;
 	struct ubi_volume *vol;