v0.21.2

Changes:

• Update Raspberry Pi OS to 2021-01-12.
• Update ws281x LED strip driver so it should work on RPi 4 Rev. 1.4.
• image: set pi3-disable-bt overlay correctly.
• All the new articles from Copter Hack 2021.
• aruco_detect: enabled dynamic reconfigure parameter added (see usage snippet).
• Add tree utility.
• Minor changes.