* Addition of I2C master driver for STM32, Generalization of ISSI3731 driver

- Addition of an i2c_master driver for STM32 to replicate expectations of AVR driver.
- Moved ISSI3731 driver one level up to make it accesible by both architectures.
- Renamed ISSI3731 functions to a more general name for preparation of other ISSI drivers.
- Added compiler directives where necessary to differenciate each architecture.

* converted tabs to spaces