pmb: introduce deviceinfo_chassis
This file holds properties like a pretty hostname and the type of
device.
See https://gitlab.com/postmarketOS/pmaports/-/merge_requests/1198#note_334342318
I've confirmed that creating /etc/machine-info on the PinePhone and installing openrc-settingsd makes the Bluetooth name be "PINE64 PinePhone" without any config file sed-ing.
I'm not sure if this "chassis" value gets used anywhere but in the future it would be nice if UIs would show "About this ${chassis}" instead of always "About this phone" or whatever. That property is also currently made optional with a fallback to "handset" (a.k.a a normal phone) but we should probably make it mandatory and retrofit all existing ports (we have phones, tablets, watches, SBCs & VMs so we use quite a few types 