targets/ulx3s and colorlight_5a_75b: cleanup USB ACM addition and only keep USB ACM changes.

- remove update in loading/flashing: we need to thinks how to integrate this.
- remove specific README: documentation is moved to the files, link to more complete project can
be added if maintained externally, as done for the iCEBreaker for example.
- revert default freq on ULX3S to 50MHz and instantiate a second PLL as done on the colorlight.