Our selftests of course rely on the kernel being built with
CONFIG_DEBUG_INFO_BTF=y, though this (nor its dependencies of
CONFIG_DEBUG_INFO=y and CONFIG_DEBUG_INFO_DWARF4=y) are not specified.
This causes the wrong kernel to be built, and selftests to similarly
fail to build.

Additionally, in the BPF selftests kconfig file,
CONFIG_NF_CONNTRACK_MARK=y is specified, so that the 'u_int32_t mark'
field will be present in the definition of struct nf_conn.  While a
dependency of CONFIG_NF_CONNTRACK_MARK=y, CONFIG_NETFILTER_ADVANCED=y,
should be enabled by default, I've run into instances of
CONFIG_NF_CONNTRACK_MARK not being set because CONFIG_NETFILTER_ADVANCED
isn't set, and have to manually enable them with make menuconfig.

Let's add these missing kconfig options to the file so that the
necessary dependencies are in place to build vmlinux. Otherwise, we'll
get errors like this when we try to compile selftests and generate
vmlinux.h:

$ cd /path/to/bpf-next
$ make mrproper; make defconfig
$ cat tools/testing/selftests/config >> .config
$ make -j
...
$ cd tools/testing/selftests/bpf
$ make clean
$ make -j
...
  LD [M]
  tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.ko
  tools/testing/selftests/bpf/tools/build/bpftool/bootstrap/bpftool
  btf dump file vmlinux format c >
  tools/testing/selftests/bpf/tools/build/bpftool/vmlinux.h
  libbpf: failed to find '.BTF' ELF section in
  vmlinux
  Error: failed to load BTF from bpf-next/vmlinux:
  No data available
  make[1]: *** [Makefile:208:
  tools/testing/selftests/bpf/tools/build/bpftool/vmlinux.h]
  Error 195
  make[1]: *** Deleting file
  'tools/testing/selftests/bpf/tools/build/bpftool/vmlinux.h'
  make: *** [Makefile:261:
  tools/testing/selftests/bpf/tools/sbin/bpftool]
  Error 2
Signed-off-by: David Vernet <void@manifault.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/bpf/20230602140108.1177900-1-void@manifault.com