Add preliminary support for HMC5883L magnetometer and MPL3115A2 altimeter.

Logging only, at this stage.

event.h

@@ -5,10 +5,12 @@
 
 #define EVENT_TIMER		0
 #define EVENT_MPU6050_I2C_COMPLETE	1
-#define EVENT_UART_INPUT	2
-#define EVENT_SDCARD		3
+#define EVENT_HMC5883L_I2C_COMPLETE	2
+#define EVENT_MPL3115A2_I2C_COMPLETE	3
+#define EVENT_UART_INPUT	4
+#define EVENT_SDCARD		5
 
-#define EVENT_MAX		3
+#define EVENT_MAX		5
 
 typedef void event_handler(void);