onie-recovery: change behaviour during recovery mode

Reviewed By: TBD

When ONIE is booted via the recovery images slightly change the
behaviour of a few utilities.

- Change the prompt to "ONIE-RECOVERY:/ #"

- Disable access to the ONIE-CONFIG partition.  During initial install
  the partitions do not exist yet.

Testing Done:

Booted the PXE EFI64 recovery image and verified the prompt changed
and that the ONIE-CONFIG partition was disabled.