- starts requirements for PX4 modules (hrt, param, etc)

 - can be reenabled after #11256 is merged

 * We have RAM problems and can't afford 4 mavlink instances. This only adds logic the rc.board for the fmu-v4 that only has an effect for SYS_AUTOSTART = 4250 (TealOne airframe).

Fixed ICM20948 magnetometer only option.
Specified bus for internal mpu9250 on Pixhawk 2.1 to avoid premature detection of
other device.

- fixes #11009

Not all boards have an SD card.

This is a *breaking* change and requires a bootloader change.

Instead of using the define __PX4_POSIX_SITL it makes more sense to have
a define just to determine if the lockstep scheduler should be used.

The new board config changed everything, so I needed to move the
lockstep_scheduler repo.