Crazyflie 2.0 nRF51 Bootloader 
Crazflie 2.0 bootloader firmware that runs in the nRF51. See readme.md in crazyflie-nrf-firmware repos for more information about flash and boot architecture.
Compiling requires the nRF51_SDK and S110 packages.
./tools/build/download_deps.sh
will download the zips and unpack them. If you want to download manually from the Nordic semiconductor website, you will find the details in nrf51_sdk/readme and s110/readme.
License
Most of the code is licensed under LGPL-3.0.
Some files under src/ble/ are modified from Nordic semiconductor examples.
Downloading the nrf51_sdk and nordic S110 softdevice require to aquire one of the Nordic Semiconductor development kit. Discussion is in progress with Nordic to solve this situation.
Compiling
To compile arm-none-eabi- tools from https://launchpad.net/gcc-arm-embedded should be in the path.