Define BUFFER_LENGTH if not already present

Default buffer size for I2C transmissions is 32 if not otherwise set.
This is a band-aid fix since there is almost certainly a more efficient
way to handle this, possibly by removing the chunking process entirely
from the I2Cdev layer if Wire implements it now based on its own
internal buffering (not sure).

Arduino/I2Cdev/I2Cdev.cpp

@@ -87,6 +87,11 @@ THE SOFTWARE.
 
 #endif
 
+#ifndef BUFFER_LENGTH
+// band-aid fix for platforms without Wire-defined BUFFER_LENGTH (removed from some official implementations)
+#define BUFFER_LENGTH 32
+#endif
+
 /** Default constructor.
  */
 I2Cdev::I2Cdev() {