Build script should only parse ATDF for selected MCUs

[Rahix]

@LuigiPiucco, so I just noticed one more thing that slipped through review ;)

In find(), you already parse all ATDFs (and also all patches). This happens before filtering for selected MCUs which means that a lot of unnecessary parsing work is done for MCUs that won't actually get code-generated.

I noticed this through suspiciously long build-times for single-MCU builds and also while playing with the error messages (injecting an error into the ATDF of an MCU that is not selected still leads to to a build error).

Do you want to take care of this?

[LuigiPiucco]

I'll need some time to work on personal things right now, so no, but I'll be back in a day or two.

[Rahix]

No worries, I did not mean for this to be urgent at all (after all, you know my response times....). Just wanted to know whether you are interested in picking this up at some point when you have time, or if you would rather see someone else put in the effort :)