efi_loader: ensure that file ubootefi.var is created

Currently file ubootefi.var is only created if the user sets a non-volatile
EFI variable. If the file is missing, a warning is written.

With the change PlatformLang is always persisted. So the file will exist on
second boot.

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