distro: Add efi pxe boot code

Now that we can expose network functionality to EFI applications,
the logical next step is to load them via pxe to execute them as
well.

This patch adds the necessary bits to the distro script to automatically
load and execute EFI payloads. It identifies the dhcp client as a uEFI
capable PXE client, hoping the server returns a tftp path to a workable
EFI binary that we can then execute.

To enable boards that don't come with a working device tree preloaded,
this patch also adds support to load a device tree from the /dtb directory
on the remote tftp server.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Tom Rini <trini@konsulko.com>