main/busybox-initscripts: use supervisor-daemon for udhcpd

Without this change udhcpd fails to start as it is compiled without the pidfile option (#11841 (closed)). While certainly possible to enable the pidfile option, I believe that we should work towards replacing pidfiles and instead rely on supervisor-daemon. Many aports (unbound, wpa_supplicant, …) already use supervisor-daemon.. I would suggest that we also start using it for busybox initscripts. This is just a start, the other scripts can be converted later on.

Fixes #11841 (closed)