Split Arty S7 25 and 50 mains.

[calder]

Before

Running

python3 -m amaranth_boards.arty_s7

defaults to S7 25. When run against an S7 50 the flashing process succeeds, but the LEDs never turn on.

After

The user has to explicitly specify an S7 version with:

python3 -m amaranth_boards.arty_s7_25

or

python3 -m amaranth_boards.arty_s7_50

Both ArtyS7_25Platform and ArtyS7_50Platform are still available for import from amaranth_boards.arty_s7.

[wanda-phi]

The general approach taken in amaranth-boards is to make the exact platform variant a command-line argument to main instead — see machxo3_sk.py for an example