efi_loader: volume name in EFI_FILE_PROTOCOL.GetInfo()

We cannot determine the volume name in U-Boot. Instead of providing a dummy
volume name in case of EFI_FILE_SYSTEM_INFO and EFI_UNSUPPORTED in case of
EFI_FILE_SYSTEM_VOLUME_LABEL consistently return an empty string.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>