I've refactored the param_parse tool to use various 'emitters'.  An emitter
can take parameter info and output it in a particular format.  Currently the
only supported emitters are the wiki and XML formats.

The goal of these changes is to create a standard machine readable description
of parameters - mainly for use by ground control stations, but it will also
enable spiffy scripting environments where code can refer symbolically to
vehicle parameters (reflectionish).

Open issue: Is there any sort of Ardupilot build id which can be included in
the generated XML?  That would ensure that we select the correct paramdefs
for the load on the target (possibly by asking the target for a SHA or
somesuch).  If that issue is resolved, then the filename for the XML
file should probably be something like:
arduplane-ca5742ac.pdef.xml.

It is worth noting that I've proposed a suffix of ".pdef.xml" for these file
types.  This facilitates automated file handling on Android devices.  On
Android you can register 'handlers' for particular file extensions and if
the user tries to open that extension in email or a web browser your app
will be given a chance to do something about it.  The 'outer' xml extension
will allow naive editors to know that at least this is an xml file.

I will include a sample of the XML format with the pull-request for this CL.

- Small optimization

ArduPPM: Changed the behaviour of the PPM encoder: if a channel is lost retain its last value instead of a fail-safe value (except for Ch3/throttle)

this has not compiled for a long time.

this brings px4 into autotest

- Allows use of hardware floating point on the Cortex-M4.
- Added "f" suffix to floating point literals.
- Call floating point versions of stdlib math functions.

this is causing some crazy results in stabilize mode. Disable it until
we sort it out

THR_FAILSAFE was combined with FS_THR_ENABLE
APPROACH_ALT was renamed to RTL_ALT_FINAL

ArduPPM: latest changes are now disabled by default, throttle low triggering by single channel errors and also recovering from a throttle low event can be enabled with defines

ArduPPM: changes for throttle low indication and also recovery after a throttle low indication are now set with defines

ArduPPM: throttle failsafe is now triggered on single channel loss but recovery from the throttle low is possible

- Make sure the values are defined as integers.  We were always using
  SPI rather than using I2C when desired.

more portable