[U-Boot] ext4 support is now merged

Hey all,

I've merged the ext4 branch (and Stephen's cache-align fix) into master and pushed. The only problem report we have currently is that for sufficiently large partitions we _may_ have unreadable directories. I'm committed to working through this issue and seeing what's going on, the main problem right now being that my large ext4 USB disk is fine and Rob's large ext4 disk (or image?) is not. ext[23] seems to be uneffected (I know the Angstrom folks have been using an older version of these patches for some time) so I believe the best course of action right now is to push this out to a wider audience.

Thanks all!