Adds a new initfs hook, verbose-initfs, which doesn't do anything on its
own. Modify postmarketos-mkinitfs to check if this hook is installed and
if so run `set -x`.

It's done this way so verbose boot is enabled as early as possible,
before hooks are loaded.

Tested on the qemu-aarch64 device.

- pmboostrap initfs hook_add verbose-initfs
- pmbootstrap qemu --arch aarch64
- confirmed that commands are echoed to the console