mavlink_ftp: fix stack overflow & add root dir prefix

- 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).