Support hooks in stage 1 init.

A process without hooks requires end user to hack the vanilla image to get the functions he needs. This is too much to do, and requires manually work again when the image is updated.

If Alpinelinux enables hooks in `init`, then above hacking could be gone. And end user just append another initrd containing the scripts to get the functions. This is much less work to do and has more chance to last between upgradings.

(from redmine: issue id 10122, created on 2019-03-16)