Enable SPI4 for FMUv2
SPI: Add SPI4 init bits for FMUv2
Fix SPI select on port 4 to use the right defines
Count devices on SPI4 / EXT from 1 as for the other buses
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
mpu6000: allow for two mpu6000 instances, one internal, one external

split g_dev into g_dev_int and g_dev_ext