efi_loader: load distro dtb in bootmgr

If no device-tree is specified, try to load a device-tree from the boot
device use the $fdtfile concatenated to either of the paths '/dtb/', '/',
'/dtb/current/'.

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