efi_loader: implement EFI_FILE_SYSTEM_INFO

Implement the information type EFI_FILE_SYSTEM_INFO in the service
GetInfo() of the EFI_FILE_PROTOCOL.

The volume label is not available in U-Boot. As a work-around use the
partition name instead.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>