Remove build for stuff dhcpcd's import-src target now does.

leot wants the ability to start wpa_supplicant from dhcpcd
without having to start wpa_supplicant seperately, thus the example
hook is restored.

Stop installing 10-wpa_supplicant.
Only install 50-ypbind when MKYP=yes.

Partially addresses PR install/54351.

Let's make things easier and split the example hooks out into
a new Makefile.