Move build script for mbed-based Arduino core to a submodule

Author: valeros

.gitmodules

@@ -1,3 +1,6 @@
 [submodule "builder/frameworks/libopencm3"]
 	path = builder/frameworks/libopencm3
 	url = https://github.com/platformio/builder-framework-libopencm3.git
+[submodule "builder/frameworks/arduino/mbed-core"]
+	path = builder/frameworks/arduino/mbed-core
+	url = https://github.com/platformio/builder-framework-arduino-core-mbed.git

builder/frameworks/arduino/arduino_h7_mbedos.py

@@ -32,9 +32,11 @@ def configure_default_packages(self, variables, targets):
 
         frameworks = variables.get("pioframework", [])
         if "arduino" in frameworks:
-            if board.startswith("portenta_h7"):
+            if board.startswith("portenta"):
                 self.frameworks["arduino"]["package"] = "framework-arduino-mbed"
-                self.frameworks["arduino"]["script"] = "builder/frameworks/arduino/arduino_h7_mbedos.py"
+                self.frameworks["arduino"][
+                    "script"
+                ] = "builder/frameworks/arduino/mbed-core/arduino-core-mbed.py"
             elif build_core == "maple":
                 self.frameworks["arduino"]["package"] = "framework-arduinoststm32-maple"
                 self.packages["framework-arduinoststm32-maple"]["optional"] = False