hostname: add hardware Stock-Keeping Unit

hostname: add hardware Stock-Keeping Unit

Add support for the hardware Stock-Keeping Unit (SKU). The SKU describes
a distinct type of hardware for sale, purchase or inventory management.
The value is read from the file /etc/machine-info or DMI as fallback.

The integration provides an unified interface to collect detail hardware
information. The /etc/machine-info entry enables embedded devices
without UEFI support to read the information from a custom store.