Add support for running the demo system on several low-cost FPGA devboards

[heliosfa]

This pull request adds support for five low-cost FPGA development boards (three of which are part of AMD's University Programme) :

• Digilent Arty S7-25 and S7-50
• Digilent Nexys A7-100T
• RealDigital Boolean
• RealDigital Blackboard

As far as I can tell, I've added all of the required config, etc. for these boards and I can successfully build and flash the bit files and load the "hello world" demo application.

[marnovandermaas]

I've tested this PR and building the bitstreams works. I don't have access to these boards so can't test them. I added a commit correcting whitespace and undid changes to vendor file because that would get overwritten if we pull in changes from upstream Ibex. Let me know if that's ok and I'll squash/rebase this PR.

[heliosfa]

Thanks for the review.

No issue with the whitespace tweaks at all - I'm a C++ dev really, so four spaces is default for me...

re. the changes to vendor, I thought they may have been needed to get the build going, but it all seems good with this commit on the Blackboard. Happy with these changes too.

[heliosfa]

One of my students found a minor issue with the Arty S7 when they tried it today, I've made some final tweaks just to make this work.