selftests/bpf: Compile kernel with everything as built-in

Since the eBPF CI does not support kernel modules, change the kernel config
to compile everything as built-in.

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Acked-by: Daniel Müller <deso@posteo.net>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20220920075951.929132-10-roberto.sassu@huaweicloud.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>