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Prior to this commit, if C2 was busy (eg lots of BLE activity) then it may
not have had time to respond to the notification on the IPCC_CH_MM channel
by the time additional memory was available to put on that buffer.  In such
a case C1 would modify the free buffer list while C2 was potentially
accessing it, and this would eventually lead to lost memory buffers (or a
corrupt linked list).  If all buffers become lost then ACL packets
(asynchronous events) can no longer be delivered from C2 to C1.

This commit fixes this issue by waiting for C2 to indicate that it has
finished using the free buffer list.

Work done in collaboration with Jim Mussared aka @jimmo.
Signed-off-by: Damien George <damien@micropython.org>