STM32H7 SPI + Exti fixes + BMI088 driver

[chris-durand]

• [x] STM32H7 SPI with DMA
• [x] Add interface to DMA driver to read transfer status
• [x] BMI088 IMU
  • [x] Driver
  • [x] SPI example
  • [x] I²C example
  • [x] Add documentation
• [x] Fix spurious EXTI interrupts with multiple enabled lines sharing the same IRQ
• [x] Fix compilation of Exti::disconnect()
• [x] Move identical copy-pasted SpiMaster::acquire()/release() code out of drivers
• [x] Allow use of SPI with fibers without :processing:protothread module