add 'NO_GLOBAL_UPDATE' define to avoid creating global UpdateClass instance (espressif#8500)

Save 180 bytes of ram if UpdateClass instance is not required

Author: vortigont

libraries/Update/src/Update.h

@@ -190,6 +190,8 @@ class UpdateClass {
  uint8_t _ledOn;
 };
 
+#if !defined(NO_GLOBAL_INSTANCES) && !defined(NO_GLOBAL_UPDATE)
 extern UpdateClass Update;
+#endif
 
 #endif

libraries/Update/src/Updater.cpp

@@ -414,4 +414,6 @@ bool UpdateClass::_chkDataInBlock(const uint8_t *data, size_t len) const {
  return false;
 }
 
+#if !defined(NO_GLOBAL_INSTANCES) && !defined(NO_GLOBAL_UPDATE)
 UpdateClass Update;
+#endif