bootm: Skip command-line substitution if !CONFIG_CMDLINE

When there is no command line, we cannot enable this feature. Add a check
to avoid a build error.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/common/bootm.c b/common/bootm.c
index dab7c36..ea71522 100644
--- a/common/bootm.c
+++ b/common/bootm.c
@@ -583,7 +583,8 @@
 		if (ret)
 			return log_msg_ret("silent", ret);
 	}
-	if (IS_ENABLED(CONFIG_BOOTARGS_SUBST) && (flags & BOOTM_CL_SUBST)) {
+	if (IS_ENABLED(CONFIG_BOOTARGS_SUBST) && IS_ENABLED(CONFIG_CMDLINE) &&
+	    (flags & BOOTM_CL_SUBST)) {
 		ret = process_subst(buf, maxlen);
 		if (ret)
 			return log_msg_ret("subst", ret);