boards: Add Vendor prefix to platforms/targets name when useful and when multiple boards from the same vendor. (With Retro-Compat on the imports).

Default to Chaser mode and similar user interface than GPIOOut.

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.

targets: keep attributes are no longer needed since automatically added when applying constraints to signals.

The system clock was driven directly while it should be driven by the PLL.

Exact PLL clock can be derived from U1 12 Mhz or X5 50 Mhz clock.