Build PACs on demand

[jbeaurivage]

This issue is just to open a discussion to see if there would be interest in building PACs on demand, instead of keeping the generated PAC code in the repo.

For reference, #665 is what prompted me to open the discussion

[rnd-ash]

Looking at this issue a few years later, I tried to get some prototype working with XRUST library to patch the SVD at compile time (Then throw it into SVD2Rust to generate the PAC), but this is way too time consuming (Seems XRUST is very slow - It takes about 10 minutes to process an SVD)

The better solution might be to change our patches to YAML format, and use svdtools library to do the SVD patching at build time.

This appears to be what is being done here https://github.com/Rahix/avr-device

[jbeaurivage]

When I tried experimenting with this, libxml and libxslt seemed to work fine without noticeable slowdown: master...jbeaurivage:atsamd:pacs-on-demand#diff-549708cc15ef1a433e42bbf1a65ca1fd82a5d33a3dadb84568e06c27549c8905