Depend on tool-mkspiffs for "buildfs" target

Author: ivankravets

platform.py

@@ -20,6 +20,7 @@ class Espressif8266Platform(PlatformBase):
     def configure_default_packages(self, variables, targets):
         if not variables.get("pioframework"):
             self.packages['sdk-esp8266']['optional'] = False
-
+        if "buildfs" in targets:
+            self.packages['tool-mkspiffs']['optional'] = False
         return PlatformBase.configure_default_packages(
             self, variables, targets)