Contribution description

FlockLab 2 is an IoT testbed with support for a variety of different hardware platforms. This PR adds a custom board definition that modifies the existing nrf52840dongle board to match how FlockLab 2 has set up their nrf52840dongle devices.

They key changes are:

• Additional GPIO definitions have been added (such as additional LEDs)
• The UART has been set to use the correct GPIO pins
• SPI and I2C have been removed as FlockLab 2 does not connect any SPI or I2C devices
• An additional compilation step has been added to support how FlockLab 2 deploys images (in elf format) to the testbed. FlockLab performs a full wipe of flash and then writes the provided image. FlockLab 2 does not load a new image with support from a boot loader. So a new rule has been added to convert the combined bin generated by riotboot back into the supported elf format. Which can be invoked by:

make BOARD=nrf52840dongle-flocklab flocklab

Testing procedure

1. Sign up for an account at https://user.flocklab.ethz.ch/
2. Build an example hello world

cd examples/hello-world
make BOARD=nrf52840dongle-flocklab flocklab

3. In examples/hello-world/bin/nrf52840dongle-flocklab there will be hello-world.flocklab.elf which can be deployed to the testbed on the nrf5 targets.
4. Deploy the image on the testbed. Note that the output may not occur due to the FlockLab logger starting up, so a short delay (e.g., 30 seconds) may need to be added to the beginning of the main function.