[Yaffs] More checkpointing

Over the last few days I've checked in some fixes and tweaks to the
checkpointing code. The last lot were to fix an issue with freespace getting
confused by a sync, and some test code to check that an existing partition
can be upgraded.

Thanks due to Kent who has been doing or coordinating a lot of the testing and
has also fixed a few bugs and put forward the qsort stuff.

At this stage I have not updated the make files, because I have been testing
with a tweaked 2.4 build and yaffs/direct, so you will have to add the new
files to the build. These are:

yaffs_qsort.c, yaffs_nand.c and yaffs_checkptrw.c

At this stage, the checkpointing is looking pretty solid and we know of no
bugs in the checkpointing stuff.

-- CHarles