[Yaffs] Add lazy laoding to yaffs2

Hi folks

I have just checked in some changes to implement lazy loading of yaffs object
details during mounting. This only applies to yaffs2 systems using extended
tags.

Lazy loading means that only extended tags info is used during the mount
scanning and further file info (name, permissions,...) are only looked up
when first needed. If suitable tags are not present then regular loading
happens instead.

This means that mounting is faster, but first directory lookup can then be a
bit slower.

There is a Kconfig entry to control this.

I have given this some testing, but not exhaustive testing.

-- Charles