stdout_serialize_begin can cause a deadlock

[peterharperuk]

See https://github.com/georgerobotics/cyw43-driver/issues/33

[kilograham]

Ok, so the problem here is interaction of two different mutexes:

core 0 is doing plain getchar() which takes stdio_usb mutex but not printf mutex.

core 1 does printf that takes printf mutex, then fails to take stdio_usb_mutex

core 0 IRQ does printf, which tries to take printf mutex

[kilograham]

merged into develop