IZipFS clean-up and bugfixing
- Move "Zip250" flag to workspace to allow running from ROM. Prior to this running the driver from ROM caused a data abort.
- Improve comments, especially in the service handler
The code is still absolutely full of magic numbers, which could do with being cleaned up properly, but it's better than it was... the way it identifies DOS format disks (reading from a fixed sector address and ignoring the MBR partition table) is likely to cause problems - but that's for another MR.