Import CrazyFlie firmware into MicroCART repo

#82 (closed) Added CrazyFlie firmware to this repo under crazyflie_software/crazyflie-firmware-2021.06 This is cloned from the official CrazyFlie release 2021.06 https://github.com/bitcraze/crazyflie-firmware/releases/tag/2021.06

The .git folder has been renamed to .git-old to make sure it doesn't conflict with current operations but to still allow reversion if needed. Submodules from the CrazyFlie repo have been added to the MicroCart repo, checked out to the same commit that the 2021.06 version uses.

Tested and compiled successfully on my machine as well as @rhunt 's crazyflie virtual machine.