esp-open-lwip: New submodule, open-source lwIP from Espressif.

Espressif releases are imported into https://github.com/pfalcon/esp-open-lwip
And that's what added as a submodule. Includes simple no-nonsense Makefile,
and going forward, may include patches or be switched to upstream lwIP with
ESP-specific patches altogether.

.gitmodules

@@ -8,3 +8,7 @@
 [submodule "esptool"]
 	path = esptool
 	url = https://github.com/pfalcon/esptool
+[submodule "esp-open-lwip"]
+	path = esp-open-lwip
+	url = https://github.com/pfalcon/esp-open-lwip
+	branch = sdk-1.5.0

esp-open-lwip

@@ -0,0 +1 @@
+Subproject commit 5bd20afa6869b682001250e2a95c261390f1098d