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

Commit 10a37fd7 "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>