Navigator: Fix RTL backtransition for VTOL

A recent change removed the command forwarding required for VTOL transitions. This change brings this back.

Partially reverts PX4#7249

FINALLY fix mag rotation issues. (PX4#7366)

* sensors : second cut at fixing mag calibration - remove old problematic code

* sensors : use more intuitive naming for loop variables

mavlink: new MAV_CMD_IMAGE_START_CAPTURE spec

The spec of the mavlink command MAV_CMD_IMAGE_START_CAPTURE has changed.

Update README

commander : simplify platform defines

BACKPORT of upstream NuttX stm32_serial: fix freezing serial port.

   Thanks to Jussi Kivilinna

   https://bitbucket.org/nuttx/nuttx/commits/9169ff6a15fe65ba4af134b470dbf89220274c19

   stm32_serial: fix freezing serial port.  Serial interrupt enable/disable functions
   do not disable interrupts and can freeze device when serial interrupt is received
   while execution is at those functions.

   Trivially triggered with two or more threads write to regular syslog stream and to
   emergency stream. In this case, freeze happens because of mismatch of priv->ie
   (TXEIE == 0) and actually enabled interrupts in USART registers (TXEIE == 1),
   which leads to unhandled TXE interrupt and causes interrupt storm for USART.

Navigator: Fix RTL command lnd logic for missions

The navigator was sending RTL commands in the wrong circumstances leading to a cycle between Navigator and Commander.