logging: add SDLOG_MODE = -1 to disable logging completely

ROMFS/px4fmu_common/init.d/rc.logging

@@ -24,4 +24,8 @@ then
 	set LOGGER_ARGS "-f"
 fi
 
-logger start -b ${LOGGER_BUF} -t ${LOGGER_ARGS}
+if ! param compare SDLOG_MODE -1
+then
+	logger start -b ${LOGGER_BUF} -t ${LOGGER_ARGS}
+fi
+

src/modules/logger/params.c

@@ -55,6 +55,7 @@ PARAM_DEFINE_INT32(SDLOG_UTC_OFFSET, 0);
  * Determines when to start and stop logging. By default, logging is started
  * when arming the system, and stopped when disarming.
  *
+ * @value -1 disabled
  * @value 0 when armed until disarm (default)
  * @value 1 from boot until disarm
  * @value 2 from boot until shutdown