810: Takes care of power for nRF USB devices r=Dirbaio a=huntc
Modifies the usb-serial example to illustrate how to setup USB for situations where the USB power can be detected and removed.
Gaps:
~~* No support for the nrf-softdevices as yet, although this should be possible via another constructor.~~
* No support for the nrf5340, although this should be possible via USBREG.
The change is tested and appears to work. Some notes:
* There's an existing field named self_powered as a UsbDevice field. It doesn't ever appear to get set. I'm wondering if this field is intended to signal that a device has the nRF VBUS power situation or not. I'm not presently using it.
* The new PowerDetected event is generated on the bus initially in situations where just new is used i.e. without power management, including on STM. We can therefore rely on this event always being generated.
Old description:
~~EnabledUsbDevice is a wrapper around the `UsbDevice` where its enablement is also subject to external events, such as `POWER` events for nRF. It is introduced generically to support other platforms should they also require external signaling for enablement.~~
Co-authored-by: huntc <huntchr@gmail.com>
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