In essence, the pxe preference doesn't apply to ppc64le.
This is because PXE data loading is part of the firmware and
does not use an intermediate boot loader like ipxe, syslinux,
or even grub.
Changes the logic to always set the stage for PXE based
deployments and enables boot setting awareness for ppc64le
based on the TFTP defaults. This allows ppc64le to coexist
with an environment preferring HTTP transport like those
used with iPXE.
Change-Id: I27fa53700c1b44f14725bc66b929732553da58f8
da503fd5ce