STM32 F4 I2c: A new implementation of the STM32 F4 I2C bottom half.  The commin I2C as this did not handled correctly in the current implementation (see also https://github.com/PX4/NuttX/issues/54).  The changes almost exclusively affect the ISR.

sscanf:  Use strtof() instead of strtod() if a short floating point value was requested.  The should help performance with MCUs with 32-bit FPU support