efi_loader: discover if no efi_system_partition is set

Variable efi_system_partition holds the efi_system_partition. Currently it
is initialized as:

    {
      .uclass_id = 0 = UCLASS_ROOT,
      .denum = 0,
      .part = 0,
    }

This indicates that host 0:0 is the efi_system_partition and we see output
like:

    => bootefi hello
    ** Bad device specification host 0 **
    Couldn't find partition host 0:0

To identify that no EFI system partition has been set use UCLASS_INVALID.

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