selftests: bpf: centre kernel bpf objects under new subdir "progs"

At the moment, all kernel bpf objects are listed under BPF_OBJ_FILES.
Listing them manually sometimes causing patch conflict when people are
adding new testcases simultaneously.

It is better to centre all the related source files under a subdir
"progs", then auto-generate the object file list.

Suggested-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>