Add driver and Python-based XVC server for Debug Bridge

[nekomona]

Hello! I have found that debugging with ILAs and VIOs are still quite handy for developing overlays, so I ported the XVC server in XAPP1251 to the PYNQ library for an easy access to the debuggers through a Debug Bridge in AXI to BSCAN config.

I have tested it with a Zynq-7000 devboard with USB RNDIS connection. It could achieve a reasonable speed for simple debugging with >500Kb on the network interface, at the cost of one core maxed out for polling during transmissions.

[codecov-commenter]

Codecov Report

Merging #1363 (6666ec0) into image_v2.8 (0cdf6b9) will decrease coverage by 0.95%. The diff coverage is 11.51%.

@@              Coverage Diff               @@
##           image_v2.8    #1363      +/-   ##
==============================================
- Coverage       46.49%   45.54%   -0.96%     
==============================================
  Files              28       29       +1     
  Lines            4955     5094     +139     
  Branches          971      996      +25     
==============================================
+ Hits             2304     2320      +16     
- Misses           2594     2717     +123     
  Partials           57       57              

Impacted Files	Coverage Δ
pynq/lib/debugbridge.py	10.86% <10.86%> (ø)
pynq/lib/__init__.py	100.00% <100.00%> (ø)

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[skalade]

Hi @nekomona, this looks like a great addition, thank you! We'll just need a bit of time to review.