AP_HAL: expose DF_LOGGING_FORMAT to user

libraries/AP_HAL/Dataflash.h

@@ -4,6 +4,13 @@
 
 #include "AP_HAL_Namespace.h"
 
+// the last page holds the log format in first 4 bytes. Please change
+// this if (and only if!) the low level format changes
+#define DF_LOGGING_FORMAT    0x28122011
+
+// we use an invalid logging format to test the chip erase
+#define DF_LOGGING_FORMAT_INVALID   0x28122012
+
 /* Dataflash abstract class:
  * This isn't the abstraction ArduPilot needs, but it is the abstraction
  * ArduPilot deserves.

libraries/AP_HAL_AVR/Dataflash_common.cpp

@@ -9,13 +9,6 @@ extern const AP_HAL::HAL& hal;
 // 0: When reach the end page stop, 1: Start overwriting from page 1
 #define DF_OVERWRITE_DATA true
 
-// the last page holds the log format in first 4 bytes. Please change
-// this if (and only if!) the low level format changes
-#define DF_LOGGING_FORMAT    0x28122011
-
-// we use an invalie logging format to test the chip erase
-#define DF_LOGGING_FORMAT_INVALID   0x28122012
-
 void CommonDataflash::erase_all() {
     for (uint16_t i = 1; i <= (_num_pages+1)/8; i++) {
         _block_erase(i);