Add PlatformIO

[ajschmidt8]

This PR replaces the st-device-sdk-c-ref build process with PlatformIO (pio).

To enable pio to build correctly, it was necessary to remove the existing CMakeLists.txt files. I also removed any component.mk files. I assume these changes are okay if we're going to adopt pio as the new build system.

Also, pio added this automatically generated sdkconfig file, which seems similar to the existing sdkconfig.defaults. I'm not sure if we'll still need the sdkconfig.defaults file. I haven't done a diff on the files to see what's different, but I imagine that any settings that need to be configured in the new file can likely be done in the new platformio.ini config file (not really sure what these files do, tbh).

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To Do:

• [ ] Update README.md
• [ ] Fix remaining build errors (not sure if they're legit, or if a build setting just needs to be changed)
• [ ] Add a GitHub Action to run a pio build on every PR (followed by a PR to migrate astyle to GitHub Actions if that's cool with you)

[ajschmidt8]

(This PR is the same as #15 except that I cut a new branch from the dev branch instead of master.)