Stop using $VERSION for the genunix label
We currently use $VERSION, which encodes the current git HEAD, as the genunix label. This is subsequently used by
kernel modules for uniquifying. The problem here is that if we make a commit, then try to rebuild a kernel module
(rather than a nightly or make in uts), it will fail, as the two labels now differ.
Now we encode the git head inside /etc/versions/build and the kernel, there is no real need to do this. Let's
replace this label with a fixed "genunix" label, so we can rebuild individual modules easily without issues.