Refactor PXE DHCP Option support.

The previous support imported pxe into driver, but driver is meant to
be entirely decoupled from specific baremetal drivers.

Co-Authored-By: Robert Collins <rbtcollins@hp.com>

Change-Id: Icc7c40fe06b520aa220906122f5f6be8d79a3ac4
Merge "Refactor PXE DHCP Option support."