- see 1259864 Data corruption in a sequence of Write-Through stores and loads
 - if we can be certain this sequence won't occur in PX4 then the d-cache will be re-enabled

 - update navigator precision landing to build without multicopter

 * The build is built with SPI4. At run time the signal GPIO_8266_GPIO2 it tested. If it is low the SPI4 is configured. If it is high SPI4 is not configured.
 * board_common: Add Notion of Board has bus manifest

ToneAlarm class refactoring to implement an interface for hardware specific methods and a single ToneAlarm class.