Add eudev to mkinitfs

Yes, I'm aware it's a quick & dirty solution to add persistent device names to mkinitfs.

However, I don't think it that unacceptable as other distros, such as Void, also uses eudev in initramfs.

Before someone takes up the work of adding this functionality to nlplug-findfs, Alpine should at least provide a way to optionally adding udev to initramfs.

This will provide support for root on ZFS for example.

Without this, the draft for OpenZFS must discourage multi-disk setup to prevent disk path swapping on reboot.