00_header.in: Do not recordfail on a read-only EXT4 FS

The EXT4 feature readonly disables the write support for a filesystem. That feature is not supported by the ext driver in GRUB.

This handles that feature by preventing the command save_env to write the whole environment, as it is done already by the other read-only filesystems.

Signed-off-by: Gaël PORTAY gael.portay@collabora.com