hostnamed: add support for getting the chassis type from device-tree

hostnamed: add support for getting the chassis type from device-tree

Device-tree based devices can't get the chassis type from DMI or ACPI,
and so far need a custom `/etc/machine-info` to set this property right.

A new 'chassis-type' toplevel device tree property has recently been
approved into the DT specification, making it possible to automate
chassis type detection on such devices.

This patch therefore falls back to reading this device-tree property if
nothing is available through both DMI and ACPI.

Signed-off-by: Arnaud Ferraris <arnaud.ferraris@collabora.com>