replace old Ethernet Arduino-library with v2, submoduled (esp8266#6812)

Author: d-a-v

.gitmodules

@@ -19,6 +19,9 @@
 [submodule "tools/esptool"]
 	path = tools/esptool
 	url = https://github.com/espressif/esptool.git
+[submodule "libraries/Ethernet"]
+	path = libraries/Ethernet
+	url = https://github.com/arduino-libraries/Ethernet.git
 [submodule "tools/sdk/uzlib"]
 	path = tools/sdk/uzlib
 	url = https://github.com/earlephilhower/uzlib.git