Can't (easily) build bpf selftest

Running tuxmake kselftest TARGETS=bpf SKIP_TARGETS="" doesn't build bpf.

I think this is because bpf is excluded by default in the selftests Makefile: https://gitlab.com/cki-project/kernel-ark/-/blob/os-build/tools/testing/selftests/Makefile#L87

And tuxmake doesn't appear to pass an empty string for SKIP_TARGETS. It appears to work if I pass a dummy string to SKIP_TARGETS.

Honestly not sure if this is a problem for TuxMake or upstream - having to pass SKIP_TARGETS="" is some pretty unintuitive behavior.