main/devicepkg-dev: Automatically install initfs-hook.sh

We have a lot of device packages that have an initfs hook so it make sense for devicepkg_package to take care of it automatically.

The following devices have an initfs hook but don't use devicepkg-dev (#8 (closed)) so they are left unchanged:

• htc-ace
• htc-bravo
• lg-d285
• lg-lenok
• motorola-osprey
• nokia-n9
• ouya-ouya