FMUv2: added defines for FMUv3 sensors

this enables EXT0 to EXT3 on external SPI bus, and gives correct names
for FMUv3 board
FMUv1: define PX4_SPI_BUS_SENSORS
SPI: make _bus protected

this allows runtime use of internal/external bus to determine if DRDY
should be used on the L3GD20
l3gd20: added -X switch for external bus
lsm303d: added -X option for external bus
mpu6000: added -X option for external bus
ms5611: added -X option for external SPI bus