Add Project XRay backend currently supporting Kintex7

[carlosedp]

This PR adds support to Project XRay supporting Xilinx series 7 FPGAs.
Currently I'm testing with Kintex 7 and a QMTech board.

The work is based on https://github.com/kintex-chatter/xc7k325t-blinky-nextpnr and is in draft (although working) state.

The backend has hard-coded paths for the databases. This is still required to be able to use Docker container flow. We need to improve this.
Managed to fix the environment variables. Works on Docker but needs testing for locally installed tools.

The supporting files to run this are at https://gist.github.com/carlosedp/a7b4386b41ffdf60873c05eeae7155ec. There is the edalize launcher script with some changes due to how quotes are handled which I plan to send a PR too.

There is also a Dockerfile to build the toolchain container image and the databases. @umarcor once it's in a more stable and/or upstream state, we can integrate to hdlc repo.

The idea is to open this for discussion.

[umarcor]

@carlosedp note that the "backend" is nextpnr in this case (VTR is supported as well). Project XRay is the "architecture documentation/database", equivalent to IceStorm, Project Trellis or Project Nexus. See:

• https://github.com/hdl/containers/blob/main/debian-bullseye/nextpnr.dockerfile
• https://github.com/hdl/containers/blob/main/.github/workflows/nextpnr.yml
• https://github.com/hdl/containers/blob/main/debian-bullseye/prjtrellis.dockerfile
• https://github.com/hdl/containers/blob/main/.github/workflows/prjtrellis.yml

[carlosedp]

Ah yes, my idea here is to create the Project Trellis equivalent for XRay where Yosys backend is used by default but since nextpnr-xilinx is kinda different from the standard nextpnr, I decided now to keep it in the xray file instead of "plugging" the nextpnr backend.

Also the build and databases is a bit different so I created the Dockerfile myself. I believe it's still early since I used forked versions from @hansfbaier that did the amazing work starting this.