Add esptool submodule.

From https://github.com/themadinventor/esptool

.gitmodules

@@ -5,3 +5,6 @@
 [submodule "lx106-hal"]
 	path = lx106-hal
 	url = https://github.com/tommie/lx106-hal
+[submodule "esptool"]
+	path = esptool
+	url = https://github.com/themadinventor/esptool

esptool

@@ -0,0 +1 @@
+Subproject commit 023314cb051de2d2d9f93e2bd14b5a10b942c5f2