Bluetooth stack only support a single device

Right now the Bluetooth stack only supports a single device. This is mostly an by-product of the lower half driver, and does not look too complex to implement.

`bt_buf_s` can be extended to include a pointer to the hardware device handling the frame. Almost all functions that make reference to `g_btdev` are working on a `bt_buf_s`. There is a slight complication around this and the kernel thread that will also need to be addressed.

While most hardware will only have a single bluetooth device, there are benefits here for configurations like the simulation where we can do a full loopback style test where the NuttX stack can connection from one attached device to another.