allow more customization for how .install files are generated

[agarwal]

@smondet mentions that more control over the generated .install file would be useful. The current implementation contains blocks of code to do various bits: generate the lines for a lib, for an app, for the META file, etc. Perhaps factor these out into their own functions, which users could then call.

Related: Also want more control over generated project.mk. However, the particular need mentioned regards just the META target. This is already deprecated and we should just remove it.