Standardize command usage messages with cmd_usage()

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
diff --git a/common/cmd_boot.c b/common/cmd_boot.c
index 6024ffe..36106c7 100644
--- a/common/cmd_boot.c
+++ b/common/cmd_boot.c
@@ -41,7 +41,7 @@
 	int     rcode = 0;
 
 	if (argc < 2) {
-		printf ("Usage:\n%s\n", cmdtp->usage);
+		cmd_usage(cmdtp);
 		return 1;
 	}