* grub-core/disk/arc/arcdisk.c (handle_writable): New var.
(reopen): New argument writable. All users updated.
Handle required access mode.
(grub_arcdisk_write): Implement.
* include/grub/arc/arc.h (grub_arc_file_access): New enum.
(grub_arc_firmware_vector): Make buffer to write a const buffer.