Makefile: Cleanup and generate help from comment

[lambdanis]

See commits.

The main goal here is to generate Makefile help from per-target doc comments instead of having all docs in the help target, which is hard to keep up-to-date. I used the approach from https://www.thapaliya.com/en/writings/well-documented-makefiles/. The result (in reality it's nicer, there are colors):

$ make help

Usage:
  make <target>

Build and install
  tetragon           Compile the Tetragon agent.
  tetragon-operator  Compile the Tetragon operator.
  tetra              Compile the Tetragon gRPC client.
  tetragon-bpf       Compile bpf programs (use LOCAL_CLANG=0 to compile in a Docker build env).
  tetragon-bench     Compile tetragon-bench tool.
  compile-commands   Generate a compile_commands.json with bear for bpf programs.
  install            Install tetragon agent and tetra as standalone binaries.

Container images
  image              Build the Tetragon agent container image.
  image-operator     Build the Tetragon operator container image.
  images             Convenience alias for image and image-operator.

Packages
  tarball            Build Tetragon compressed tarball.
  tarball-release    Build Tetragon release tarball.

Test
  check              Run Go linters.
  test               Run Go tests.
  tester-progs       Compile helper programs for unit testing.
  bpf-test           Run BPF tests.
  verify             Verify BPF programs.
  alignchecker       Run alignchecker.
  bench              Run Go benchmarks.
  test-compile       Compile unit tests.
  e2e-test           run e2e tests
  e2e-test E2E_BUILD_IMAGES=0  run e2e tests without (re-)building images
  e2e-test E2E_TESTS=./tests/e2e/tests/skeleton  run a specific e2e test

Development
  cscope             Generate cscope for bpf files.
  kind               Create a kind cluster for Tetragon development.
  kind-install-tetragon  Install local version of Tetragon in the kind cluster.
  kind-setup         Create a kind cluster and install local version of Tetragon.

Chores and generated files
  protogen           Generate code based on .proto files.
  crds               Generate kubebuilder files.
  vendor             Tidy and vendor Go modules.
  clang-format       Run code formatter on BPF code.
  go-format          Run code formatter on Go code.
  format             Convenience alias for clang-format and go-format.
  generate-flags     Generate Tetragon daemon flags for documentation.
  metrics-docs       Generate metrics reference documentation page.
  lint-metrics-md    Check if metrics documentation is up to date.

Documentation
  help               Display this help, based on https://www.thapaliya.com/en/writings/well-documented-makefiles/
  docs               Preview documentation website.
  version            Print Tetragon version.

CLI
  cli-release        Compile tetra CLI release binaries.

Options
  TARGET_ARCH             target architecture to build for (e.g. amd64 or arm64)
  BPF_TARGET_ARCH         target architecture for BPF progs (set by TARGET_ARCH)
  GO_ARCH                 target architecture for Go progs (set by TARGET_ARCH)
  DEBUG                   enable NOOPT and NOSTRIP
  NOOPT                   disable optimization in Go build (set by DEBUG)
  NOSTRIP                 disable binary stripping in Go build (set by DEBUG)
  LOCAL_CLANG             use the local clang install for BPF compilation
  JOBS                    number of jobs to run for BPF compilation (default to nproc)
  EXTRA_GO_BUILD_LDFLAGS  extra flags to pass to the Go linker
  EXTRA_GO_BUILD_FLAGS    extra flags to pass to the Go builder
  EXTRA_TESTFLAGS         extra flags to pass to the test binary

[kkourt]

LGTM, thanks!

[mtardy]

This is awesome