The tmcuart_send command increases cpu usage on the micro-controller.
Should multiple tmcuart_send commands be issued at the same time to a
single AVR micro-controller, it could increase the load to the point
that it introduces a failure. It could also lead to tmcuart_send
transmission errors, which would cause retransmission requests, which
further increase the load.
Track and share mutexes so that only one tmcuart_send command can be
active on a single mcu at a time.
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>