disk: allow - or empty string to fall back to $bootdevice

Commit 10a37fd "disk: get_device_and_partition() "auto" partition"
prevented the use of "-" on the command-line to request fallback to the
$bootdevice environment variable instead. This patch allows that, or an
empty string "" to be used.

Tested:
setenv bootfile /boot/zImage
setenv bootdevice 0:1
ext2load mmc 0:1
ext2load mmc -
ext2load mmc ""

Signed-off-by: Stephen Warren <swarren@nvidia.com>

disk/part.c

@@ -489,7 +489,8 @@ int get_device_and_partition(const char *ifname, const char *dev_part_str,
 	disk_partition_t tmpinfo;
 
 	/* If no dev_part_str, use bootdevice environment variable */
-	if (!dev_part_str)
+	if (!dev_part_str || !strlen(dev_part_str) ||
+	    !strcmp(dev_part_str, "-"))
 		dev_part_str = getenv("bootdevice");
 
 	/* If still no dev_part_str, it's an error */