Fix HTTPUpdate flash size check and add SPIFFS size check (#2161)

* Fix error in PR #2048: if ::available() is called before ::connect() _rxBuffer is not initialised

* Fixed flash size check and added SPIFFS size check

* Rewriting ESP.getFreeSketchSpace(), moving code from HTTPUpdate.cpp

libraries/HTTPUpdate/src/HTTPUpdate.h

@@ -42,6 +42,7 @@
 #define HTTP_UE_SERVER_FAULTY_MD5           (-105)
 #define HTTP_UE_BIN_VERIFY_HEADER_FAILED    (-106)
 #define HTTP_UE_BIN_FOR_WRONG_FLASH         (-107)
+#define HTTP_UE_NO_PARTITION                (-108)
 
 enum HTTPUpdateResult {
     HTTP_UPDATE_FAILED,