DataFlash: add EnableWrites method

This method allows temporarily suspending writes to the dataflash which
will be used in ArduCopter to ensure no messages are written while the
copter is disarmed

libraries/DataFlash/DataFlash.h

@@ -42,6 +42,7 @@ public:
 
     /* logging methods common to all vehicles */
     uint16_t StartNewLog(void);
+    void EnableWrites(bool enable) { _writes_enabled = enable; }
     void Log_Write_Format(const struct LogStructure *structure);
     void Log_Write_Parameter(const char *name, float value);
     void Log_Write_GPS(const GPS *gps, int32_t relative_alt);
@@ -74,6 +75,7 @@ protected:
 
     const struct LogStructure *_structures;
     uint8_t _num_types;
+    bool _writes_enabled;
 
     /*
       read a block

libraries/DataFlash/DataFlash_Block.cpp

@@ -37,7 +37,7 @@ void DataFlash_Block::FinishWrite(void)
 
 void DataFlash_Block::WriteBlock(const void *pBuffer, uint16_t size)
 {
-    if (!CardInserted() || !log_write_started) {
+    if (!CardInserted() || !log_write_started || !_writes_enabled) {
         return;
     }
     while (size > 0) {
@@ -168,6 +168,7 @@ void DataFlash_Block::EraseAll()
     StartWrite(df_NumPages+1);
     uint32_t version = DF_LOGGING_FORMAT;
     log_write_started = true;
+    _writes_enabled = true;
     WriteBlock(&version, sizeof(version));
     log_write_started = false;
     FinishWrite();

libraries/DataFlash/DataFlash_File.cpp

@@ -142,7 +142,7 @@ void DataFlash_File::EraseAll()
 /* Write a block of data at current offset */
 void DataFlash_File::WriteBlock(const void *pBuffer, uint16_t size)
 {
-    if (_write_fd == -1 || !_initialised) {
+    if (_write_fd == -1 || !_initialised || !_writes_enabled) {
         return;
     }
     uint16_t _head;

libraries/DataFlash/DataFlash_SITL.cpp

@@ -106,6 +106,9 @@ void DataFlash_SITL::BlockWrite(uint8_t BufferNum, uint16_t IntPageAdr,
                                 const void *pHeader, uint8_t hdr_size,
                                 const void *pBuffer, uint16_t size)
 {
+    if (!_writes_enabled) {
+        return;
+    }
     if (hdr_size) {
         memcpy(&buffer[BufferNum][IntPageAdr],
                pHeader,

libraries/DataFlash/LogFile.cpp

@@ -13,6 +13,7 @@ void DataFlash_Class::Init(const struct LogStructure *structure, uint8_t num_typ
 {
     _num_types = num_types;
     _structures = structure;
+    _writes_enabled = true;
 }