Comment 16 for bug 1917609

Nice catch on that cause 670b0872ad88 ("UBUNTU: SAUCE: selftests:
Skip BPF seftests by default"), in which we make the bpf subset as
a default skip target by default. (This explained why this works in comment #13)

I prefer to keep TARGETS=bpf and add an empty SKIP_TARGETS= to the
make command to override the default value, so that we can still
align with how we start a kernel-selftests subset (like in the
ubuntu_kernel_selftests).

e.g.
utils.make('-C linux/tools/testing/selftests TARGETS=bpf SKIP_TARGETS= clean all')

A SKIP_TARGETS= might be helpful for pepole to know this is for
overriding the default value as well.

Thanks
Sam