rtems-yaffs: Checkpoint the filesystem at unmount time

Adding a checkpoint greatly reduces the amount of time required to mount
the filesystem on the next boot.  After a clean unmount, remount time
is only a second or few.  After an unclean reboot, remount time requires
a scan of the filesystem, which can take a few minutes.

Signed-off-by: Charles Manning <cdhmanning@gmail.com>