selftests: net: extract BPF building logic from the Makefile

selftests: net: extract BPF building logic from the Makefile

The BPF sample building code looks a little bit spaghetti-ish
so move it out to its own Makefile snippet. Similar in the spirit
to how we include lib.mk. libynl will soon get a similar snippet.

There is a small change hiding in the move, the relative
paths (../../.., ../.. etc) are replaced with variables
from lib.mk such as top_srcdir and selfdir.

Link: https://lore.kernel.org/r/20240423183542.3807234-3-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>