Fix STM32 I2C but for 1-byte Reads.
Hacked the driver to work, but does not conform
to datasheet behavior.

The STM32 I2C drivers needs to be rewritten from scratch.
Use xpcc::SoftwareI2C in the meantime.
Add working STM32 I2C driver.
Not all tricks are fully documented yet.
Beautify code and remove much debugging helpers.
Remove busy waiting for stop condition.
Add I2C sensor example code to the STM32.
Add documentation of operation and configuration.
Merge branch 'feature/stm32_i2c_debug' into develop
Circumvent issue when compiling constexpr with avr-gcc.
Also "fix" the avr i2c thermometer example.
AVR ADC docu typos fixed
Copy from blink example to exti
Working example of external interrupt
External Interrupts for GPIO pins. 

Tested with SMT32 f4 Discovery board with PA0 and PE11.
STM32 EXTI: Compile for F1 and others, too
Typo ADC
STM32 EXTI: Compile for F2
AVR ATtiny2313 does not have an ADC.
Moved init/reset sequence to project.cfg. openocd.cfg is now only configuration.
Doxygen group + docu for SAB2