Add caching for generated files

[dergraaf]

Files should not be regenerated if the configuration did not change.

[salkinium]

I'm not sure how much time this will save tho. For reference, building node tree for lbuild discover for a large modm example takes ~0.7s, generating the the selected modules takes ~1.8s.

Ok, generating the whole library takes ~5s. Good point.

[salkinium]

This needs to be super robust though. There is nothing worse than to start mistrusting lbuild because it didn't rebuild something.

[dergraaf]

That is true. The solution I had in mind would actually not bring any speed-up (or only if we're IO write limited), but would instead allow to detect local changes by hashing/comparing the output before writing it to the output file. This would solve #19 as well.