deviceinfo: remove external_disk_install and external_disk, use external_storage instead

This checks the next box in #688 (closed)

Removed deviceinfo_external_disk_install and deviceinfo_external_disk variables in deviceinfo files and use deviceinfo_external_storage instead.

Also adapted the related python code and tests to work with the new variable.

TODO when the PR will be closed:

• update the https://wiki.postmarketos.org/wiki/Deviceinfo_reference page