boards: enable flow control on CDCACM

This enables flow control on CDCACM for the NuttX boards which fixes a
problem where HITL would stall.

The stall could happen if the hardware would be a bit too slow in
keeping up with the incoming messages. Often, this happened on arming
because the logger would take some time to log all parameters right at
the beginning.

The stall would then not recover due to NuttX bug where the rx interrupt
would not be restored correctly and instead only a slower watchdog would
release the next read. This watchdog takes 200ms which means it's hard
to impossible to get out of the situation without restarting sim and/or
PX4.  For more information about the issue, see:
apache/incubator-nuttx#3633

As a workaround, until that bug is fixed, and because it makes sense
anyway, I propose to enable FLOWCONTROL for the serial via USB.

boards/spracing/h7extreme/nuttx-config/nsh/defconfig

@@ -47,6 +47,7 @@ CONFIG_BOARD_RESET_ON_ASSERT=2
 CONFIG_BUILTIN=y
 CONFIG_C99_BOOL8=y
 CONFIG_CDCACM=y
+CONFIG_CDCACM_IFLOWCONTROL=y
 CONFIG_CDCACM_PRODUCTID=0x004b
 CONFIG_CDCACM_PRODUCTSTR="PX4 SP RACING H7 EXTREME"
 CONFIG_CDCACM_RXBUFSIZE=600