AP_HAL_Linux: document functions to write file

Document function and add attributes to mark then as printf- and
scanf-like functions.

libraries/AP_HAL_Linux/Util.h

@@ -2,7 +2,9 @@
 #ifndef __AP_HAL_LINUX_UTIL_H__
 #define __AP_HAL_LINUX_UTIL_H__
 
+#include <AP_Common/AP_Common.h>
 #include <AP_HAL/AP_HAL.h>
+
 #include "AP_HAL_Linux_Namespace.h"
 #include "ToneAlarmDriver.h"
 
@@ -40,8 +42,19 @@ public:
 
     uint32_t available_memory(void) override;
 
-    int write_file(const char *path, const char *fmt, ...);
+    /*
-    int read_file(const char *path, const char *fmt, ...);
+     * Write a string as specified by @fmt to the file in @path. Note this
+     * should not be used on hot path since it will open, write and close the
+     * file for each call.
+     */
+    int write_file(const char *path, const char *fmt, ...) FORMAT(3, 4);
+
+    /*
+     * Read a string as specified by @fmt from the file in @path. Note this
+     * should not be used on hot path since it will open, read and close the
+     * file for each call.
+     */
+    int read_file(const char *path, const char *fmt, ...) FMT_SCANF(3, 4);
 
 private:
     static Linux::ToneAlarm _toneAlarm;