create capability for specifying custom maps folder

In my workflow, it would be very nice to set up many different maps folders and then direct Geopaparazzi to each, depending on need.   @moovida, I've copied all your "custom sdcard path" code and reworked it for setting custom maps folders. I've tested on two units and it seems to work.  The only glitch I wish I knew how to fix is that you need to restart geopap after changing the settings.  

If you think this might be useful, I would be thrilled if you considered adding it. If this is a bad time, this can wait and I can rebase again after you bring in the large branches you both are working on! And, if you'd rather not have this capability, that's ok too!  We will use it in our fork.

[[Note that the custom SDcard path functionality seems quirky and I could not get geopap to run on a real external SD card - there seemed to be a database access issue. I did not look into that any further. I THINK this is completely independent of this pull request.]]