Support Multiple FPGA Bitfiles - Avoid Collisions

[JamesWiresmith]

We need to make sure if there are multiple bitfiles that we avoid collisions in the software.

For the registers etc. everything is prefaced so that is not a problem.

However we need to make sure the ni_fpga functions are only compiled once. There are two ways I can see this working:

1. We extend the build API to be aware of multiple FPGAs.
2. We split the compiles into a common object and specific objects and assume that it wont error if we try to compile the common twice - or detect that it exists and don't compile again.

I think either method could work quite nicely.