This avoids a confusing ENOMEM raised from gap_advertise if there is
currently an active connection. This refers to the static connection
buffer pre-allocated by Nimble (nothing to do with MicroPython heap
memory).
On other ports (e.g. ESP32) they provide a complete Nimble implementation
(i.e. we don't need to use the code in extmod/nimble). This change
extracts out the bits that we don't need to use in other ports:
- malloc/free/realloc for Nimble memory.
- pendsv poll handler
- depowering the cywbt
Also cleans up the root pointer management.
