build/lattice/trellis: make "-abc9" an optional argument

[gsomlo]

Fix up earlier commit (#6c298cb7) and make the '-abc9' optional
argument to yosys' synth_ecp5 actually optional (and off by default)
in LiteX's trellis build infrastructure.

@enjoy-digital @daveshah1 What do you think? IMHO I should have done it like this from the beginning, any reason we shouldn't correct that now? :)

[mithro]

I'm pretty sure abc9 should be on by default and bugs reported where it performs worse when off?

[gsomlo]

abc9 is not on by default in yosys, so maybe making it on by default in litex was, um, "premature" ? Right now, about half the time, yosys fails with abc9 (when I build rocket-based litex socs for versa or the trellisboard). I reported it upstream here: berkeley-abc/abc#84 At the moment, the proposed solution (berkeley-abc/abc#89) is applied in the Fedora package, but doesn't seem to actually fix abc9 failures reliably. The upstream abc maintainer has not (yet) responded.

[enjoy-digital]

Thanks @gsomlo, i also had trouble with this and also agree to use the same default settings than Yosys.

[enjoy-digital]

@gsomlo: for info, abc9 was also causing issue with bus-standard=axi-lite on ECP5 designs, without it it's now fine, so this confirms it's probably better to disable it by default.