postmarketos-mkinitfs: allow hooks in multiple places

Right now, we can only execute all hooks at one point in the initramfs. In the discussion of https://gitlab.com/postmarketOS/charging-sdl/issues/20 ("Should we split the charging functionality into a dedicated hook?"), we came to the conclusion that we need hooks in multiple cases.

This may sound complex, but it can be done with a few lines of code actually:

https://gitlab.com/postmarketOS/charging-sdl/issues/20#note_85260841