mro_x21-777: defconfig MMCSD enable multiblock
- ardupilot bootloader compatibility (first 3 sectors) - clock tree fixes - start mavlink - start mpu9250 magnetometer (only onboard mag) - fixes https://github.com/PX4/Firmware/issues/15466
- this doesn't currently change anything, but gets us ready to start experimenting with using the small amount of instruction tightly memory on STM32F7 - the .ramfuncs section works with NuttX CONFIG_ARCH_RAMFUNCS