loader - guard against empty rootdev

At least one instance of u-boot pretending to be EFI
is passing empty rootdev to loader which does not end well.

A simple precaution is harmless.

Reviewed by:	imp
Differential Revision:	https://reviews.freebsd.org/D50334

stand/common/metadata.c

@@ -111,7 +111,7 @@ md_load_dual(char *args, vm_offset_t *modulep, vm_offset_t *dtb, int kern64)
      * tested/set by MI code before launching the kernel.
      */
     rootdevname = getenv("rootdev");
-    if (rootdevname == NULL)
+    if (rootdevname == NULL || *rootdevname == '\0')
 	rootdevname = getenv("currdev");
     /* Try reading the /etc/fstab file to select the root device */
     getrootmount(rootdevname);

stand/efi/loader/main.c

@@ -513,7 +513,7 @@ find_currdev(bool do_bootmgr, bool is_last,
 	 * it's wrong.
 	 */
 	rootdev = getenv("rootdev");
-	if (rootdev != NULL) {
+	if (rootdev != NULL && *rootdev != '\0') {
 		printf("    Setting currdev to configured rootdev %s\n",
 		    rootdev);
 		set_currdev(rootdev);