bpf: Add kernel/modules BTF presence checks to bpftool feature command

This adds both the CONFIG_DEBUG_INFO_BTF and CONFIG_DEBUG_INFO_BTF_MODULES
kernel compile option to output of the bpftool feature command.

This is relevant for developers that want to account for data structure
definition differences between kernels.

Signed-off-by: Grant Seltzer <grantseltzer@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20210222195846.155483-1-grantseltzer@gmail.com