Merge ubuntu-cdimage:lp.2063471 into ubuntu-cdimage:main

This will work and is certainly not harmful, but I'm a bit against doing such very specific special-casing in the main checksum code. Are we certain that the only cases we will see for netboot are the ones requiring re-packing of the tarball? Looking at the code... it seems so. If you checked that that's the case, then it's a +1. Otherwise, I'd prefer to have a bit more sublime cases - like checking if we *actually* did modify the netboot tarball.

I suspect that it's the case however, as the url is an essential part of the tarball from what I recall?

considerations here:
- the information about whether a netboot tarball is being repacked is *very* far removed in the code from where we do the copying of checksums; plumbing this through would be very ick in terms of APIs
- yes, every time in the code that we pass old_dirs to checksum_directory, it's in a context where we would have repacked the netboot tarball
- the use of "old_directories" for checksum_directory is an optimization, to avoid having to recompute checksums over large files (i.e. installer images). The netboot tarballs are on the order of 80MB each, trivial in comparison if there ever were a case where we were needlessly re-checksumming.

Okay, I guess this is good enough then!