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.
- cmake NuttX build wrapper compile in place instead of copying source tree to build directory
- slightly faster skipping necessary copying (depending on system)
- allows debugging in place
- easier to work directly in NuttX following official documentation
- simplifies overall build which should make it easier to resolve any remaining NuttX dependency issues in the build system
- the downside is switching back and forth between different builds always require rebuilding NuttX, but I think this is worth the improved developer experience
- also no longer builds px4io and bootloader in every single build, for most users these rarely change and we're wasting a lot of build time
- no longer start sercon or mavlink usb by default
- on USB connection (VBUS) monitor serial USB at low rate and start Mavlink if there's a HEARTBEAT or nshterm on 3 consecutive carriage returns
- the mavlink USB instance is automatically stopped and serdis executed if USB is disconnected
- skipping Mavlink USB (and sercon) saves a considerable amount of memory on older boards
