[Yaffs] New background garbage collector

Hi All

I have just pushed the new background garbage collection feature to git.
http://www.yaffs.net/gitweb?p=yaffs2/.git;a=commit;h=36dc48ebac4140345b3f5955d5013f6c22ad827a

This has had some testing but I'll be doing more over the next few days.

Background garbage collection should significantly reduce foreground gc in
many usage scenarios, particularly those where writing is "bursty" such as a
cell phone or similar applications. This could make writing a lot faster,
making applications more responsive.

Feedback welcome.

-- Charles