tools: bpftool: add probes for kernel configuration options

Add probes to dump a number of options set (or not set) for compiling
the kernel image. These parameters provide information about what BPF
components should be available on the system. A number of them are not
directly related to eBPF, but are in fact used in the kernel as
conditions on which to compile, or not to compile, some of the eBPF
helper functions.

Sample output:

    # bpftool feature probe kernel
    Scanning system configuration...
    ...
    CONFIG_BPF is set to y
    CONFIG_BPF_SYSCALL is set to y
    CONFIG_HAVE_EBPF_JIT is set to y
    ...

    # bpftool --pretty --json feature probe kernel
    {
        "system_config": {
            ...
            "CONFIG_BPF": "y",
            "CONFIG_BPF_SYSCALL": "y",
            "CONFIG_HAVE_EBPF_JIT": "y",
            ...
        }
    }

v5:
- Declare options[] array in probe_kernel_image_config() as static.

v4:
- Add some options to the list:
    - CONFIG_TRACING
    - CONFIG_KPROBE_EVENTS
    - CONFIG_UPROBE_EVENTS
    - CONFIG_FTRACE_SYSCALLS
- Add comments about those options in the source code.

v3:
- Add a comment about /proc/config.gz not being supported as a path for
  the config file at this time.
- Use p_info() instead of p_err() on failure to get options from config
  file, as bpftool keeps probing other parameters and that would
  possibly create duplicate "error" entries for JSON.

v2:
- Remove C-style macros output from this patch.
- NOT addressed: grouping of those config options into subsections
  (I don't see an easy way of grouping them at the moment, please see
  also the discussion on v1 thread).

Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>