I don't remember the reason why you didn't end up using the "normal" imu_aspirin_v2.1 subsystem/module (where the mag is a slave of the MPU)...
In any case with the conversion from subsystems to modules  we decided to not provide different defaults for LPC and STM based boards anymore, as that was causing quite a bit of confusion.
So now we only have one default (See http://docs.paparazziuav.org/v5.10/module__imu_mpu6000_hmc5883.html
), in that case the default IMU_MPU_SPI_SLAVE_IDX is now SPI_SLAVE0 for all boards (where formerly it was SPI_SLAVE2 only for STM boards).
And as SPI_SLAVE2 doesn't exist on all boards, SLAVE0 was a better default...
And you don't have to change the module xml file, you can configure it from your airframe file:
<configure name="IMU_MPU_SPI_SLAVE_IDX" value="SPI_SLAVE2"/>