Merge branch 'selftests-net-extract-bpf-building-logic-from-the-makefile'

Jakub Kicinski says:

====================
selftests: net: extract BPF building logic from the Makefile

This has been sitting in my tree for a while. I will soon add YNL/libynl
support for networking selftests. This prompted a small cleanup of
the selftest makefile for net/. We don't want to be piling logic
for each library in there. YNL will get its own .mk file which can
be included. Do the same for the BPF building section, already.

No funcional changes here, just a code move and small rename.
====================

Link: https://lore.kernel.org/r/20240423183542.3807234-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>