Add MPU9150 mag support and mag autodetection. Add AK8975 mag driver.

Note: not working on the sparky, MPU9150 needs passthough enabling but
when enabled the mag and gyro won't ack on the default addresses.  Needs
further investigation.

src/main/drivers/sensor.h

@@ -0,0 +1,21 @@
+/*
+ * This file is part of Cleanflight.
+ *
+ * Cleanflight is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * Cleanflight is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with Cleanflight.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+typedef void (*sensorInitFuncPtr)(void);                    // sensor init prototype
+typedef void (*sensorReadFuncPtr)(int16_t *data);           // sensor read and align prototype