make yosys version string compatible with what nmigen expects

[edbordin]

Currently because I changed the version string format, nmigen can't parse it and ignores our yosys build. Standard yosys version string is something like 0.9+3292 which can be updated with make bumpversion

[edbordin]

I'll revise this so that it doesn't run bumpversion as per whitequark's advice in YosysHQ/yosys#2207. Will also pass GIT_REV in to the yosys makefile since it runs after .git has been stripped away (strangely git rev-parse seems to be outputting something anyway).