("Master files") Warthog-Stick.xml and Warthog-Throttle.xml support
configuration overlay files. These are optional and can be used to
modify the default configuration. Warthog-Stick.xml searches for
overlay files in

  {$FG_HOME,$FG_ROOT}/Input/Joysticks/ThrustMaster/Warthog/Stick/

and Warthog-Throttle.xml in

  {$FG_HOME,$FG_ROOT}/Input/Joysticks/ThrustMaster/Warthog/Throttle/


Available files are loaded in the following order, where <x> is replaced
by the contents of property "x":

  - generic.xml
  - helicopter.xml  (only helicopters)
  - </sim/type>.xml  (some aircraft set this property to "seaplane")
  - </input/joysticks/overlay>.xml  (not used by the system; meant to
               be set on the command line or in wrapper scripts,
               for example: --prop:input/joysticks/overlay=modern_jet)
  - </sim/aircraft>.xml  (aircraft name, for example "harrier.xml")


If a file is found under $FG_HOME, then an equally named file under
$FG_ROOT will be ignored.

Overlay files are like stripped down joystick driver files, but shall
not have a <name> entry. They only need to define what should differ
from the already loaded joystick configuration. Make sure that an
axis definition uses the same index as the master file. The n-th
<axis> in an XML config file usually has index n, even if the axis
uses <number><unix> etc. Nasal blocks are properly executed in the
namespace of the master file, so you can access all its functions
and variables.