efi_loader: fix append write behavior to non-existent variable

efi_loader: fix append write behavior to non-existent variable

Current "variables" efi_selftest result is inconsistent
between the U-Boot file storage and the tee-based StandaloneMM
RPMB secure storage.

U-Boot file storage implementation does not accept SetVariale
call to non-existent variable with EFI_VARIABLE_APPEND_WRITE,
it return EFI_NOT_FOUND.
However it is accepted and new variable is created in EDK II
StandaloneMM implementation if valid data and size are specified.
If data size is 0, EFI_SUCCESS is returned.

Since UEFI specification does not clearly describe the behavior
of the append write to non-existent variable, let's update
the U-Boot file storage implementation to get aligned with
the EDK II reference implementation.

Signed-off-by: Masahisa Kojima <kojima.masahisa@socionext.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Tested-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>