[plugins] Change default plugin directory

• The default directory can now be on the USB stick so plugins are
  preserved with fresh SD images.

• We can now do this since we invoke the callbacks using our eventScript
  wrapper.  Previously we couldn't do this because the callback
  functionality wasn't able to be marked executable on the USB stick
  which is formatted FAT32.

NOTE: This breaks if the plugin does anything to modify the base
image.
Merge branch 'move_usb_dir'
[settings] Populate our settings array as well as the individual global variable.
[plugins] Update VastFMT plugin for a bug fix.