RFC: mass storage with write support

[tyll]

I would like to change the mass storage example to also allow write support with storing the actual changes in RAM. Since the RAM is quite limited in tomu, this would require to reduce the actual size of the mass storage device to a size that is not enough for a vfat filesystem. The current example implements a read-only vfat filesystem, though. Do you have any preference about how to implement this change? My idea was to make this conditional on a define so one can select to either get the current example or a fully working, quite slow mass-storage device (4 to 7 kiB probably).

[xobs]

Making it a compile-time switch is probably easiest.

We can ship prebuilt versions of both the readonly and read/write version in the prebuilt/ directory.