pb-discover: add dtb support for PXE config

pb-discover: add dtb support for PXE config

Currently there is no way to manually specify a DTB file when with a PXE
network boot configuration file. This makes it difficult when you need
to work with or emulate a special snowflake machines with special snowflake
hardware.

Some ARM systems provide this feature with the "fdt" option so this patch
adds support for using the ftd or dtb configuration options to the PXE
config parser.

Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>