HAL_AVR: change order of CS pin init - fixes APM2 boot problem

We now init the MPU6k CS pin before the MS5611 CS pin. This should not
matter at all, but it turns out that it solves the APM2 boot
problem. We should investigate why, as this may indicate an electrical
problem.

Pair-Programmed-With: Pat Hickey

libraries/AP_HAL_AVR/SPIDeviceManager_APM2.cpp

@@ -11,20 +11,28 @@ using namespace AP_HAL_AVR;
 extern const AP_HAL::HAL& hal;
 
 void APM2SPIDeviceManager::init(void* machtnichts) {
-   
-    /* ms5611 cs is on Arduino pin 40, PORTG1 */
-    AVRDigitalSource* ms5611_cs = new AVRDigitalSource(_BV(1), PG);
-    /* ms5611: divide clock by 32 to 500khz
-     * spcr gets bit SPR1,  spsr gets bit SPI2X */
-    _ms5611 = new AVRSPI0DeviceDriver(ms5611_cs, _BV(SPR1), _BV(SPI2X));
-    _ms5611->init();
 
-    /* ms5611 cs is on Arduino pin 53, PORTB0 */
+    /* Note that the order of the init() of the MS5611 and MPU6k is
+     * critical for the APM2. If you initialise in the wrong order
+     * then the MS5611 doesn't initialise itself correctly. This
+     * indicates an electrical fault in the APM2 which needs to be
+     * investigated. Meanwhile, initialising the MPU6k CS pin before
+     * the MS5611 CS pin works around the problem
+     */
+
+    /* mpu6k cs is on Arduino pin 53, PORTB0 */
     AVRDigitalSource* mpu6k_cs = new AVRDigitalSource(_BV(0), PB);
     /* mpu6k: divide clock by 32 to 500khz
      * spcr gets bit SPR1,  spsr gets bit SPI2X */
     _mpu6k = new AVRSPI0DeviceDriver(mpu6k_cs, _BV(SPR1), _BV(SPI2X));
     _mpu6k->init();
+
+    /* ms5611 cs is on Arduino pin 40, PORTG1 */
+    AVRDigitalSource* ms5611_cs = new AVRDigitalSource(_BV(1), PG);
+    /* ms5611: divide clock by 32 to 500khz
+     * spcr gets bit SPR1,  spsr gets bit SPI2X */
+    _ms5611 = new AVRSPI0DeviceDriver(ms5611_cs, _BV(SPR1), _BV(SPI2X));
+    _ms5611->init();
    
     /* optflow cs is on Arduino pin A3, PORTF3 */
     AVRDigitalSource* optflow_cs = new AVRDigitalSource(_BV(3), PF);