The xPi's usually don't have a VPD so the chassis mac-address probed at
boot is usually null in /run/system.json. This commit adds a fallbkack
mechanism to populate this field so it can be used for unique hostnames
even on these boards.
Ths ietf-hardware.yang model does not have a notion of physical address,
so we augment one tht is generic enought to be used for other hardware
components than Ethernet, similar to what ietf-interfaces.yang use.
Signed-off-by: Joachim Wiberg <troglobit@gmail.com>