Setting static ip= for baremetal PXE boot.

This patch adds a new option "pxe_network_config" to baremetal PXE driver,
if set it tells the driver to append the network configuration to the
kernel cmdline. Initramfs built with network support could then benefit
from this option and avoid the injection of network configuration files
into the image.

DocImpact

Change-Id: I61dc946f42f8a86eaad0e73b033a0d8da4a74e40
Merge "Setting static ip= for baremetal PXE boot."