Note there was one place where a non-strict prototype was actually being used
with an argument, in `zmk_hog_init`. In this case, the actual argument type was
added instead.
Added some utility functions and an event for tracking the state of the
USB connection.
Updated endpoints.c to select a single endpoint to send HID reports to
based on the status of the USB and BLE connections. Partially fixes#206.
Future commits will add a user setting to control which endpoint is used if
both USB and BLE are ready.