meta-lxatac-software/labgrid: Export USB-Ports as Power Switches

[SmithChart]

All three USB ports on the LXA TAC are power switchable (e.g. using uhubctl). labgrid already has a driver that can handle ports where power can be switched: USBPowerPort.

With this change we'll export the three USB ports as power ports. This way they can be used to switch DUTs on or off.

The resources exported will look something like this:

$ labgrid-client r
lxatac-00013/dut_power/NetworkPowerPort
lxatac-00013/lxatac-usb-ports-power-p1/NetworkUSBPowerPort  # <-- new
lxatac-00013/lxatac-usb-ports-power-p2/NetworkUSBPowerPort  # <-- new
lxatac-00013/lxatac-usb-ports-power-p3/NetworkUSBPowerPort  # <-- new
lxatac-00013/serial/NetworkSerialPort

[hnez]

I've always assumed that we would implement this using the HTTP API of the tacd but it does not have to be that way. Using uhubctl is perfectly fine, as it uses the same kernel API as the tacd to do the switching and the tacd notices the change eventually via periodic polling.

[SmithChart]

Implementing this using the HTTP API shouldn't be any more difficult than using the USBPowerPort. But a polling interval of 1s still feels quite responsive.