Add partition scheme for mpc85xx devices

[jhibbits]

Summary

Some mpc85xx devices with u-boot need MBR partitioning with a FAT boot
partition. Since the infrastructure is already in place to have a dedicated
boot partition, this adds the necessary bits to use that infrastructure with
mpc85xx boards.

[jhibbits]

Eliminate an extraneous parenthesis pair, add the mpc85xx check to create a
512MB boot partition.