This uses the app_simple_timer library, which uses the high frequency clock
for keeping track of time.
In reality, we want to use the low frequency clock with RTC, since it's much
more power efficient and we don't need high precision.
Currently uploading via the Arduino IDE and PlatformIO works. But it's not perfect yet:
1. We're using a custom-compiled bootloader for the e73 board, which is nice;
2. We're still using `board = adafruit_feather_nrf52832` in the `platformio.ini` file. Using our custom board `boards/e73-tbb.json` doesn't work yet. Uploading results in a `*** [upload] IndexError : list index out of range` error. Most likely has to be fixed in https://github.com/platformio/platform-nordicnrf52/blob/develop/builder/main.py.
