v1.12.0

This is the v1.12 stable release, which brings a number of significant improvements to PX4:

The full release notes are here: https://docs.px4.io/master/en/releases/1.12.html

* Multi-EKF enabled by default
* Safety (switch) defaults to off (motors are disarmed, but servos / flaps can move)
* Safety switch is latching: Once it is disabled, it will stay disabled
* Multicopter
   * [Intuitive stick feel in Position mode](https://docs.px4.io/master/en/releases/1.12.html#intuitive-stick-feel-in-position-mode)
   * [Hover thrust independent velocity control gains](https://docs.px4.io/master/en/releases/1.12.html#hover-thrust-independent-velocity-control-gains)
* UAVCANv0: Although the fundamental features like Firmware upgrades and parameter sync of CAN nodes have been implemented for over 5 years, we refreshed support now that finally, devices are on the market. Typical CAN GPS, airspeed and power modules are supported
* UAVCANv0 Node: PX4 supported building nodes for many years - now we support building specific targets like the CUAV GPS units
* UAVCANv1: Initial alpha of a complete end-to-end implementation
* Fixedwing/VTOL significant TECS improvements
* Magnetometer calibration faster and more robust
    * new soft iron calibration coefficients 
    * automatically determine the rotation of external sensors
* Gyro dynamic notch filtering with onboard FFT
* Optimized rate control sensor pipeline (minimal inner loop end-to-end latency)
* Added support for IRC Ghost including telemetry
* new board support
   * PX4 FMUv6u
   * PX4 FMUv6x
   * CUAV X7/X7Pro
   * CUAV Nora
   * CUAV CAN GPS
   * SP Racing H7 Extreme
   * Bitcraze Crazyflie v2.1
   * ARK CAN Flow
   * ARK CAN GPS
   * Freefly CAN RTK GPS
   * mRo Ctrl Zero H7 (experimental)