Path saving sim: use the common pios_flashfs for the simulation

The lower driver is not connected yet so this will crash.  We will
use the code from the test directory.
Path saving simulation: connect the ut filesystem to sim

Must copy theflash.bin from unit tests to your working simulation
directory or there will be a failure to start simulator.

Fixes #482
Path saving: fix bug in erasing old waypoints
Path saving: stop saving at any invalid waypoints