Standardize command usage messages with cmd_usage()

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
diff --git a/common/command.c b/common/command.c
index a4a978c..c396648 100644
--- a/common/command.c
+++ b/common/command.c
@@ -379,6 +379,11 @@
 	return find_cmd_tbl(cmd, &__u_boot_cmd_start, len);
 }
 
+void cmd_usage(cmd_tbl_t *cmdtp)
+{
+	printf("Usage:\n%s\n", cmdtp->usage);
+}
+
 #ifdef CONFIG_AUTO_COMPLETE
 
 int var_complete(int argc, char *argv[], char last_char, int maxv, char *cmdv[])