Watrli plant node

[PeterKietzmann]

This PR introduces a test for the moisture sensor. Basically the following is done in this PR:

• introduce UART_1
• change default STDOUT from UART_0 to UART_1
• import and adapt the ADC functionalyity to a mode with external reference voltage
• add a test for the moisture sensor

Please note that you need to connect an external tty/USB converter to the UART_1 pins so see the STDOUT

[PeterKietzmann]

Compare [1] or just the last commits to see the actual changes

[https://github.com/PeterKietzmann/RIOT/tree/watrli_plant_node]

[Lotterleben]

I've updated our fork to reduce the number of foreign commits and make your contributions more clear, but some have apparently stuck around... are those commits that aren't in the master which you needed & merged into the fork?

[PeterKietzmann]

@Lotterleben, I don't get exactly what you mean :/ . However, if you wonder about the great numer of commits (unrelated to my changes), this is because our RIOT-masters differ in state. I rebased just KNOW :-)