Prior to this commit, BTstack would only be notified of sent UART data when
the mp_bluetooth_hci_poll() function was called for some other reason, eg
because of incoming data over UART. This is highly suboptimal.
With this commit, BTstack is now notified immediately after UART data has
been sent out. This improves the multi_bluetooth/perf_gatt_char_write.py
performance test by about a factor of 10x for write-without-response, and
about 4x for write-with-response (tested on LEGO_HUB_NO6 as instance1).
Signed-off-by: Damien George <damien@micropython.org>
