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

There seems to be some capacitance on KEY+, so the usual 115200 don't work

ECP5-EVN board: Added BUTTON_1 and SPI flash pins to IOs

- Deprecate Python 3.5, switch to Python 3.6+.
- Remove which was not used or updated. We'll see to get this back when working on releases.

programming the ECP5-EVN flash through the OpenOCD JTAG-SPI proxy

zcu104: add fully working SO-DIMM config