UART EasyDMA example

[jamolnng]

This example echos commands, denoted by a new line, sent to the UARTE0 interface and over RTT (Real-Time Transfer) I/O protocol for debug purposes. I saw there were a few other pull requests with UART examples but they were more complicated, this gives a starting point which is severely under-documented.

This was designed and tested on the nRF52840-DK (PCA10056): https://www.nordicsemi.com/Software-and-Tools/Development-Kits/nRF52840-DK

but I'm sure it can be easily expanded