Reunite `litex-buildenv` with LiteX platforms

[cr1901]

To reduce the amount of code duplication we should subclass from litex platforms' _io, connectors and methods instead of redefining them. This will prevent surprises and subtle incompatibilities between litex-buildenv and litex platforms if a user of litex-buildenv decides they want to use litex (or even migen) directly for their current project.

Contrived example: Maybe a user of litex wants complete customization for their shiny new project?

List of existing platforms:

• [ ] arty.py
• [ ] atlys.py
• [ ] mimasv2.py
• [ ] minispartan6.py
• [ ] netv2.py
• [ ] nexys_video.py
• [ ] opsis.py
• [ ] pipistrello.py

Platforms to add:

• [ ] arty_s7.py
• [ ] tinyfpga_b.py

The following are possibly okay leaving in litex-buildenv/not subclassing from LiteX?

• [ ] sim.py
• [ ] tofe_axiom.py
• [ ] tofe_lowspeedio.py

[mithro]

There are more boards here too -> https://github.com/timvideos/HDMI2USB-litex-firmware/pull/398

• [ ] Digilent CMOD A7 (just platform file added)
• [ ] Numato Lab Galatea Spartan 6 PCIe Development Board
• [ ] Numato Lab Neso Artix 7 100T FPGA Module
• [ ] Numato Lab Saturn Spartan 6 LX45 FPGA Module

[mithro]

@piotr-binkowski