Made weak reference to shutdown BT stack, to avoid linking it if not needed

[chipguyhere]

I noticed that when I use the POWER class to shut down the M5Stack Core device, my flash requirements grew by over 600KB.

I discovered that this was because there's a call to shutdown the BT stack as part of the process that brings current consumption to a minimum. But I'm not using the BT stack, so shutting it down is not necessary, and having it present in my binary is counterproductive.

This change makes the call to shutdown the BT stack into a weak reference, so it only gets called if the BT stack is already linked, but without linking it if it's not otherwise being used.

[Tinyu-Zhao]

I would like to ask if you use platformIO to determine the flash occupied when building the project?