efi_loader: fix BootOrder variable measurement handling

UEFI specification does not require that BootOrder is defined.
In current implementation, boot variable measurement fails and
returns EFI_NOT_FOUND if BootOrder is not defined.

This commit correcly handles this case, skip the boot variable
measurement if BootOrder is not defined.

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