Only set flashmode byte when uploading an executable

Fixes esp8266#6880

The updater was patching in the proper flashmode configuration byte for
all uploads, apps and filesystems.  This ended up corrupting one byte on
every FS upload.

Change updated to only patch theflashmode if it is doing an app.

Author: earlephilhower

cores/esp8266/Updater.cpp

@@ -329,7 +329,7 @@ bool UpdaterClass::_writeBuffer(){
   bool modifyFlashMode = false;
   FlashMode_t flashMode = FM_QIO;
   FlashMode_t bufferFlashMode = FM_QIO;
-  if (_currentAddress == _startAddress + FLASH_MODE_PAGE) {
+  if ((_currentAddress == _startAddress + FLASH_MODE_PAGE) && (_command == U_FLASH)) {
     flashMode = ESP.getFlashChipMode();
     #ifdef DEBUG_UPDATER
       DEBUG_UPDATER.printf_P(PSTR("Header: 0x%1X %1X %1X %1X\n"), _buffer[0], _buffer[1], _buffer[2], _buffer[3]);