nimble_port_initialized` is false by default and is set to true as soon as
Adds a variable that indicates whether `nimble_port_init` has been called by `nimble_riot_init`, i.e. that the NimBLE stack has been initialized. The variable can be used to decide whether events from the low-level BLE controller driver should be forwarded to the NimBLE stack. It is necessary to avoid crashes in the case that the higher-prioritized thread of the low-level BLE controller driver starts sending events to the host before the NimBLE stack has been initialized by the lower-prioritized host thread.
