build-sys: add make 'help' target

Add a make 'help', to print a summary of the main Makefile targets.
The format is loosely inspired by Linux make 'help' output.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20160913142033.7705-1-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>