remove bpftool binaries and create a bpftool submodule

[rafaeldtinoco]

Currently we have 2 bpftool binaries in order to generate the tailored BTF files. Instead of having binaries in the repo, now that bpftool has a GitHub repository:

https://github.com/libbpf/bpftool

We can simply have that repository as a submodule and compile bpftool as part of the process of using it.

[brycekahle]

One downside to this, is that you need a full setup to be able to build bpftool everywhere you want to run btfhub.

[rafaeldtinoco]

Done by:

#95
#96

Unfortunately, with the mentioned downside. The bpftool compilation is agnostic to distro (depends on available -dev packages so it can be built).

Maybe I should create a docker container to compile both: bpftool and pahole statically, and then install the result binaries where we're running btfhub. This would give us a way to always have both updated whenever we want (oracle/suse/rhel/debian/ubuntu).