[libre-riscv-dev] HDL selection

[lkcl]

ok!  so cpu_decoder.py was relatively straightforward:
https://git.libre-riscv.org/?p=rv32.git;a=blob;f=cpu_decoder.py

i took the opportunity to do a tidyup, split down into function calls,
added short docstrings for each function and class, and it's pretty
obvious what's going on: it's just a combinatorial logic block with
some if and case statements, and bit manipulation.

i think this is one of the files that could actually be used, it's
very straightforward.

l.