MicroPython and NimBLE must be on the same core, for synchronisation of the
BLE ringbuf and the MicroPython scheduler. However, in the current IDF
versions (3.3 and 4.0) there are issues (see e.g. #5489) with running
NimBLE on core 1.
This change - pinning both tasks to core 0 - makes it possible to reliably
run the BLE multitests on esp32 boards.
This prevents issues with concurrent access to the ringbuf.
MICROPY_BEGIN_ATOMIC_SECTION is only atomic to the same core. We could
address this with a mutex, but it's also not safe to call mp_sched_schedule
across cores.
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).
