Use espressif Arduino branch "master"

Author: Jason2866

platform.py

@@ -88,7 +88,7 @@ def configure_default_packages(self, variables, targets):
             self.packages["framework-arduinoespressif32"]["optional"] = False
             self.packages["framework-arduinoespressif32-libs"]["optional"] = False
             # use latest espressif Arduino libs
-            URL = "https://raw.githubusercontent.com/espressif/arduino-esp32/release/v3.1.x/package/package_esp32_index.template.json"
+            URL = "https://raw.githubusercontent.com/espressif/arduino-esp32/master/package/package_esp32_index.template.json"
             packjdata = requests.get(URL).json()
             dyn_lib_url = packjdata['packages'][0]['tools'][0]['systems'][0]['url']
             self.packages["framework-arduinoespressif32-libs"]["version"] = dyn_lib_url