efi_loader: load device-tree specified in boot option

We allow to specify the triple of binary, initrd, and device-tree in boot
options.

Add the code to actually load the specified device-tree.

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