- update to NuttX with stm32f4 and stm32f7 SPI DMA internal buffers
- remove explicit DMA buffer allocations from new IMU drivers
- restore original BOARD_DMA_ALLOC_POOL_SIZE
- decrease SPI DMA thresholds based on fmu-v2/v3/v4/v5 bench testing
- uses the FIFO and SPI DMA to transfer full raw data (8 kHz gyro, 4 kHz accel)
- new sensor messages for better visibility
- sensor_{accel, gyro}_fifo: full raw data for optional logging and analysis
- sensor_{accel, gyro}_status: metadata, clipping, etc
- currently not enabled by default