device/*/deviceinfo: remove empty variables

Fixes #574 (closed) by warning huawei-alice: Please remove the empty variable: deviceinfo_dtb=""

I would have liked to reuse info but pmb.parse.deviceinfo() seems to add variables which aren't defined. Resulting in false-positives i.e. dev_touchscreen being "" for the pinephone.
Should I rather change the behavior of deviceinfo()?

Is this the right file to add the check?