main/busybox: enable mdev daemon support

It would be nice to enable support for running busybox' mdev as a daemon.

This will allow init images using mdev to not need the kernel uevent helper anymore, as its use is discouraged. From UEVENT_HELPER config help in the linux kernel:

This should not be used today, because usual systems create many events at bootup or device discovery in a very short time frame. One forked process per event can create so many processes that it creates a high system load, or on smaller systems it is known to create out-of-memory situations during bootup.

This is needed to unlock this MR from postmarketOS: https://gitlab.com/postmarketOS/pmaports/-/merge_requests/3351