Add deviceinfo to initramfs

It was mentioned in https://github.com/postmarketOS/pmbootstrap/issues/88#issuecomment-318635417 and other places I don't remember now.

The deviceinfo file contains variables that may be useful for initramfs too. At the moment this file is usually smaller than 1KB.

This way the variables can be used in initramfs and we don't have to replace them in place.