Allow the ubldr boot device to be choosen by a u-boot environment variable.

If a "loaderdev=<device>" env variable is set and the named device
exists, it is used.  If the device doesn't exist, fall back to the
historic "probe" loop that prefers disk devices over network devices.

If the env var is not set, preserve the historic behavior of using the
first working disk device provided by u-boot, or a network device if no
functional disk device is found and a network device exists.

The old probe loop is reworked so that it checks all bootable devices
provided by u-boot rather than taking an early-out on the first device
found.  This results in the cosmetic change of listing all potential boot
devices for the user, but the behavior of which device it chooses is the
same as it has always been.