getInitfsExtraFiles: add scripts from hooks-extra

Put scripts from /etc/postmarketos-mkinitfs/hooks-extra into the extra initramfs instead of the regular one, similar to how it is possible with files listed in /etc/postmarketos-mkinitfs/files-extra.

This way we will be able to launch hooks not only very early in the initramfs as it's currently the case. But also later on after the initramfs-extra was extracted, and more files are available. ondev2 will make use of this feature.