Allow the u-boot loaderdev env var to be formatted in the "usual" loader(8)

Allow the u-boot loaderdev env var to be formatted in the "usual" loader(8)
way: device<unit>[s|p]<slice><partition>.  E.g., disk0s2a or disk3p12.
The code first tries to parse the variable in this format using the
standard disk_parsedev().  If that fails, it falls back to parsing the
legacy format that has been supported by ubldr for years.

In addition to 'disk', all the valid uboot device names can also be used:
mmc, sata, usb, ide, scsi. The 'disk' device serves as an alias for all
those types and will match the Nth storage-type device found (where N is
the unit number).