This adds an optional feature to trigger an action if a specific key is
held when the keyboard is powered on. It can be configured to jump to
the bootloader and/or clear settings.
This is inspired by QMK's "bootmagic lite" feature, and it is primarily
intended as a way to recover a keyboard which doesn't have a physical
reset button in case it is flashed with firmware that doesn't have a
&bootloader key in its keymap. It can also be used to clear BLE bonds on
the peripheral side of a split keyboard without needing to flash
special firmware.
