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.
* New ZMK_SLEEP Kconfig symbol to enable the functionality.
* Switch to PORT events that allows wake from deep sleep.
* Initial basic power management policy, with idle ms,
and ignoring deep sleep if we detect a USB connection.