fix(usb): detect USB power mode to fallback to BLE

[angweekiat]

Update zmk_usb_is_hid_ready to check that HID configuration from the host has been done, before allowing the keyboard to be used as a HID device.

This is done by adding bool is_configured and toggling it off when the device is disconnected, and enabling it when usb_status == USB_DC_CONFIGURED.

This is a naive solution for #841, where having a USB-power cable without data driving the keyboard, like a power bank, still has the firmware thinking that it's connected to a host and futilely sending data over USB.

When the cable to a power source is plugged in, the USB DC status (from Zephyr) goes through the following states:

USB_DC_CONNECTED -> USB_DC_SUSPEND

While for cables to a host machine:

USB_DC_CONNECTED -> USB_DC_SUSPEND -> USB_DC_RESUME -> USB_DC_RESET -> USB_DC_CONFIGURED

The status code list can be found at https://github.com/zephyrproject-rtos/zephyr/blob/main/include/zephyr/drivers/usb/usb_dc.h#L33-L58

In the case of a power bank being plugged in, the code changes above allow zmk_usb_is_hid_ready to declare that HID isn't ready yet, and thus allows endpoints.c to fallback to BLE connection.

Steps to reproduce the original issue

1. (Setup) Have ZMK keyboard with Bluetooth profile enabled
2. Set the keyboard to prefer sending to USB (&out OUT_USB)
3. Power on the keyboard via USB via a power bank
4. Type, text should not show up anywhere ('consumed' by power bank)

Testing

• Built and flashed the firmware with the commit changes, ran the same steps above, and text showed up on the expected device.
• Tested using 2 laptops (Ubuntu / Windows) using 2 different power banks.
• Tested for regression, keyboard still works as expected for waking up the host machine (CONFIG_USB_DEVICE_REMOTE_WAKEUP=y) and for boot protocol (CONFIG_ZMK_USB_BOOT=y).

Concerns

• I can't find any USB or USB HID specifications that explicitly state that the host machine will do the USB configuration step before the USB HID connection is considered complete.
• It's my first time contributing to ZMK, please let me know how the pull request can be improved!

[petejohanson]

Thanks for the PR!

One major question about testing/statuses for another scenario.

[petejohanson]

Finally had a chance to give this a test on a real device. Works like a champ from my few test scenarios. Thanks so much for the PR! I am going to suggest we leave some of our docs around this issue as-is for now, pending further real world testing with your change in place.