ivory-px4-tests sensorsmonitor is more resilient to malformed packets

got one when rebooting the board, made a nasty crash
ivory-bsp-stm32: arbitrarily make I2C and SPI higher priority than UART

SPI has a bug where it will hang forever if preempted, apparently. I haven't
tracked down the exact bug yet.
This makes a crash less likely to happen, though not impossible.
Will work out a proper fix soon.