efi_loader: Fix to set bootdev_root correctly if bootdev found

Fix find_boot_device() to set bootdev_root if it finds the
bootdev from BootNext. Currently it sets the bootdev_root only
when it finds bootdev from BootOrder.

Fixes: c74cd8bd ("efi_loader: capsule: add capsule_on_disk support")
Signed-off-by: Masami Hiramatsu <masami.hiramatsu@linaro.org>
Accked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>