fix global , and allow easy subclassing of Platform

[zignig]

The global in the tinyfpga_bx board was an integer not a string. Added extend to platform to make subclassing and adding resources easier.

[codecov]

Codecov Report

Merging #81 into master will decrease coverage by 0.02%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master      #81      +/-   ##
==========================================
- Coverage      81%   80.97%   -0.03%     
==========================================
  Files          31       31              
  Lines        4876     4879       +3     
  Branches     1050     1050              
==========================================
+ Hits         3950     3951       +1     
- Misses        799      801       +2     
  Partials      127      127

Impacted Files	Coverage Δ
nmigen/build/plat.py	24.5% <33.33%> (+0.13%)	⬆️

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[whitequark]

I fixed the typo in 45d1dc1. As for your other change, I don't think it's necessary. There is nothing wrong with using normal Python mechanisms like super(), and you could write it in even shorter way like this:

class MyOwnPlatform(TinyFPGAPlatform):
    resources = TinyFPGAPlatform.resources + [
        Resource(...)
    ]