device-raspberry-pi*: fix boot error caused by missing dtb files

The upstream linux-rpi package has been modified: the dtb files are now located in /boot/dtbs.

This is the same fix done in !870 (merged) applied to the other RPI models.

I've also symlinked the post-upgrade script to the post-install, so that the dtb files are updated during a package upgrade.

I didnt' tested it on the real devices but I verified that each package correctly install the dtb files in the /boot folder.

EDIT by ollieparanoid: removed closes... since #452 (closed) needs to stay open, as the pi 2 is still broken.