Export root "env" to the extra script

docs/projectconf.rst

@@ -486,9 +486,7 @@ Example, specify own upload command for :ref:`platform_atmelavr`:
 
 .. code-block:: python
 
-    from SCons.Script import DefaultEnvironment
-
-    env = DefaultEnvironment()
+    Import('env')
 
     env.Replace(UPLOADHEXCMD='"$UPLOADER" ${ARGUMENTS.get("custom_option")} --uploader --flags')
 

platformio/builder/main.py

@@ -128,7 +128,7 @@ if "UPLOAD_FLAGS" in env:
     env.Append(UPLOADERFLAGS=["$UPLOAD_FLAGS"])
 
 if env.get("EXTRA_SCRIPT"):
-    env.SConscript(env.get("EXTRA_SCRIPT"))
+    env.SConscript(env.get("EXTRA_SCRIPT"), exports="env")
 
 if "envdump" in COMMAND_LINE_TARGETS:
     print env.Dump()