The CDC layer on F1, F3 and F4 now always acts like an
infinte data sink whenever *either* there is no DTE present
(ie. no terminal program listening) *or* the USB cable is
disconnected.
F1 and F4 were previously checking the cable but not the DTE.
F3 didn't check anything. The COM layer didn't even ask the
lower layers.
All of this used to mean that any time a caller did a blocking
send to a CDC device without a DTE, it would eventually block
for up to a 5s timeout waiting for space in the Tx buffer.
Conflicts:
flight/PiOS/STM32F30x/pios_usb_cdc.c
