ONIE environment commands: onie-env-set, onie-env-get

Introduce ONIE commands for manipulating the persistant ONIE state
variables.

For now, on powerpc with U-Boot the new commands onie-env-set,
onie-env-get are simply wrappers around fw_setenv and fw_printenv
respectively.

Testing Done:

Built images and verified that the new commands behave the same as the
original fw_setenv and fw_printenv commands.

Reviewed By: Vidya Ravipati <vidya@cumulusnetworks.com>
Signed-off-by: Curt Brune <curt@cumulusnetworks.com>