Also use new loose lookup_request to simplify constraints.

platforms: make sure all Xilinx/Altera platforms have a create_programmer method, use OpenOCD on Spartan6 and 7-Series.

As discussed recently on Discord.

- 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.

Addition of USB ACM for ECP5

arty_s7: fix copyrights, rename to arty_s7, various minor changes to make it similar to others targets.

Added Arty S7 board

As the pin-out is totally different from the A7 board I did put this
in a separate class and not as a variant of the Arty board.
Used migen Arty S7 board file and Digilent xdc file as reference.

Improve compatability with colorlight 5a-75b v6.1