SerialFlash: enlarge chip ID buffer

SerialFlashChip::readID() writes up to 5 bytes into
the buffer provided, but the caller only allocates
3 bytes. Increase buffer size to 5 bytes.

SerialFlashDirectory.cpp

 	// last check, if enough space exists...
 	len = strlen(filename);
 	// TODO: check for enough string space for filename
-	uint8_t id[3];
+	uint8_t id[5];
 	SerialFlash.readID(id);
 	if (address + length > SerialFlash.capacity(id)) return false;