Add help section to make file

Being used to other build systems i naturally type "make help" when i forget the name of the make targets, which doesn't work.

I know this can also be done with Make <space> <tab><tab>, but that requires bash-completions to be installed also.

I feel this could help people less familar with Make get a head start on building things.

Example output:

 $  make help
all
bootstrap
build
build-tar
build-vm
check-abi
check-dev-files
checkout/x86_64/vm/minimal-systemd-vm.bst
check-rpath
clean
clean-platform
clean-repo
clean-runtime
clean-test
clean-vm
export
export-docker
export-oci
export-snap
export-tar
list
manifest
markdown-manifest
repo
runtimes/x86_64-desktop-platform-image
run-vm
track-mesa-aco