efi_loader: Setup default location for UEFI Variables storing

efi_loader: Setup default location for UEFI Variables storing

EFI_VARIABLE_FILE_STORE is only available when FAT_WRITE is enabled but
that's not valid for all platforms and dependency should be covered.

Also Kconfig behavior is that if default option is not valid then Kconfig
selects the first presented valid option instead hence it is better to
record EFI_VARIABLE_NO_STORE as safe default option.

Suggested-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Michal Simek <michal.simek@amd.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>