Make "tool-mbctool" package optional

It's made to isolate users from possible broken packages in briki manifest

platform.json

@@ -79,6 +79,7 @@
     }, 
     "tool-mbctool": {
       "type": "uploader",
+      "optional": true,
       "version": ">=2.0.0"
     },
     "tool-openocd-esp32": {

platform.py

@@ -41,6 +41,7 @@ class Espressif32Platform(PlatformBase):
         if build_core == "mbcwb":
             self.packages['framework-arduinoespressif32']['optional'] = True
             self.packages['framework-arduino-mbcwb']['optional'] = False
+            self.packages['tool-mbctool']['optional'] = False
 
         return PlatformBase.configure_default_packages(self, variables,
                                                        targets)