This can help "unbrick" AeroFC when a bad firmware is loaded
and it keeps rebooting or it spinning in some loop.

No need to request to stay in booloader as it will stay
in bootloader because the pin is set.

The linux targets don't have it and setting a fixed constant causes
wrong vehicle associationss in Flight Review.

This is needed by some consumers like transponders.

This reverts commit dde57811.

- change memory allocation from stack to a malloc'd buffer. This avoids
  increasing the stack size. And since FTP is rarely used, the buffers
  are only allocated upon use and freed after a time of 2s inactivity.
- adds PX4_ROOTFSDIR as root directory prefix. This does not change
  anything on NuttX, but in SITL it will avoid enumerating the whole
  disk tree when using QGC (which enumerates all files recursively).

The provided argument payload->offset is in range [0, num_file_entries-1],
but seekdir might use a completely different range. It is not defined
by the API spec. It is only useful in conjunction with telldir().

To reduce flash space usage

This can happen for example when a param type is changed from int32 to
float. The type check will then fail if the param is stored and the param
import will be aborted.
Now we just skip the entry and continue loading the rest.

This will ensure that no transponder reports are dropped and that all received reports are passed on to the GCS and other devices.

With the previous value of 20ms, we dropped RC input when not armed. The
result was that some RC channels jumped randomly between min & max.

This closes the fd and cleans up a static variable used in the dsm parser.

This was only a problem when running as a task not on the work queue.
The problem was that init() opened the RC serial device, which was then
read in the main loop, which is a different context when run as a task.