- 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).