Update floating point calculations to use floats instead of doubles.

- 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.
Add AVR compatibility header for missing math.h definitions.

- Define float versions of math functions to the double versions
  on AVR (eg. #define sinf sin).
- These macros appear to be missing in older versions of avr-libs.
- Include AP_Math.h rather than math.h to get these definitions.