update Makefile and add help information. exec make help will print e…

[hurricane1988]

make help

Usage:
  make <target>

General
  help             Display this help.

Development
  fmt              Run go fmt against code.
  vet              Run go vet against code.

Build
  build-exporter   Build exporter binary.
  build-skoop      Build skoop binary.
  build-collector  Build collector binary.
  build-controller  Build controller binary.
  build-btfhack    Build btfhack binary.
  build-webconsole  Build webconsole binary.

Dependencies
  generate-bpf     Generate bpf.
  generate-bpf-in-container  Generate bpf in container.

[CLAassistant]

All committers have signed the CLA.

[jzwlqx]

nice work! LGTM