hci_stack is currently static, making it impossible to access it from outside code.

This is bad for applications where the application needs to be aware of the information available inside hci_stack_t. It is also bad for systems where it is possible/necessary to use more than one HCI BT devices. (for example, embedded USB host that supports multiple BT dongles attached to a hub)

The first step to resolving this problem is to make hci_stack not static, and provide an extern access to it.

To make it easier to support multiple HCI, simply add a hci_stack_t pointer as an argument to all functions, and encapsulate all static variables into the hci_stack_t structure.