cmd: eficonfig: add support for setting fdt

cmd: eficonfig: add support for setting fdt

We already support creating a load option where the device-path
field contains the concatenation of the binary device-path and
optionally the device path of the initrd which we expose via the
EFI_LOAD_FILE2_PROTOCOL.

Allow to append another device-path pointing to the device-tree
identified by the device-tree GUID.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>