commander: check magnetometers for inconsistency preflight

src/modules/commander/commander_params.c

@@ -590,6 +590,18 @@ PARAM_DEFINE_FLOAT(COM_ARM_IMU_ACC, 0.7f);
  */
 PARAM_DEFINE_FLOAT(COM_ARM_IMU_GYR, 0.25f);
 
+/**
+ * Maximum magnetic field inconsistency between units that will allow arming
+ *
+ * @group Commander
+ * @unit Gauss
+ * @min 0.05
+ * @max 0.5
+ * @decimal 2
+ * @increment 0.05
+ */
+PARAM_DEFINE_FLOAT(COM_ARM_MAG, 0.15f);
+
 /**
  * Enable RC stick override of auto modes
  *