[2.0.x] Compile only HAL for selected PIO environment

[fiveangle]

Any interest in this @thinkyhead ?

• Reasonable speed-up on builds (which should further reduce travis build time)
• Adds a small level of compile failure isolation when specific platform builds break
• Consolidates "shared" HAL files so they aren't littered in the main HAL directory (will hopefully keep additional "shared" resources from accumulating in different directories)

[fiveangle]

Hold up as it appears my rebase picked up a few rreverts. Will fix them up when back.

EDIT: All fixed up

[thinkyhead]

I do like the idea. I can try to rebase it after applying some persistent store patches.