- 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.
We initially wanted to provide different level of support for the platforms/targets, mainly
to avoid too much maintenance and let each contributor update its contributed platforms and
targets, but it's easier to update all platforms/targets all-together when LiteX evolves or
changes (and that's what has been done on litex-boards since the creation of the repository).
So let just simplify things and avoid this differentiation.