Print help with list of targets by default

[ondrej-fabry]

Running make (without specifying any target) or make help will now print list of documented make targets.

➤ make
List of make targets:
agent                          Build agent
install                        Install commands
cmd                            Build commands
clean-cmd                      Clean commands
examples                       Build examples
clean-examples                 Clean examples
debug-remote                   Debug remotely
test                           Run unit tests
test-cover                     Run unit tests with coverage
perf                           Run quick performance test
perf-all                       Run all performance tests
integration-tests              Run integration tests
generate                       Generate all
generate-proto                 Generate Go code for Protobuf files
generate-binapi                Generate Go code for VPP binary API
verify-binapi                  Verify generated VPP binary API
generate-desc-adapters         Generate Go code for descriptors
dep-check                      Check Go dependencies
lint                           Lint Go code
format                         Format Go code
get-linkcheck                  Check links in Markdown files
yamllint                       Lint YAML files
images                         Build all images
dev-image                      Build developer image
prod-image                     Build production image

This is done by parsing Makefile and listing make targets that contain special comment, e.g.:

build: ## Builds agent
    ...