Allow disabling the BPF build process
Draft because it's just a quick typing-out.
This re-adds -Dbpf=userhacks
to enable/disable the build of userhacks - default enabled. Then we simply skip over any BPF object that isn't enabled which means that by passing -Dbpf=[]
we can disable the BPF build altogether.
The main change in functionality however is that previously we always built the stable
BPFs (but didn't install them) whereas now we don't build those unless explicitly specified.
Intended to replaces parts of !148