runqueue: Ensure task environment is correct

[Richard Purdie <richard.purdie@linuxfoundation.org>]

This fixes two problems:

a) Variables which were in the parent environment but not set as "export"
   variables in the datastore could end up in the task environment

b) oe.environ.update() can't cope with the generator returned by 
   bb.data.exported_vars()

Whilst the updated code isn't as neat, it does do the expected thing,
sets the environment correctly and stops unwanted values leaking into
the task environment.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

diff --git a/bitbake/lib/bb/runqueue.py b/bitbake/lib/bb/runqueue.py
index 5a4321f..72e6845 100644
--- a/bitbake/lib/bb/runqueue.py
+++ b/bitbake/lib/bb/runqueue.py
@@ -1136,7 +1136,12 @@ class RunQueueExecute:
                 for h in self.rqdata.hash_deps:
                     the_data.setVar("BBHASHDEPS_%s" % h, self.rqdata.hash_deps[h])
 
-                os.environ.update(bb.data.exported_vars(the_data))
+                # exported_vars() returns a generator which *cannot* be passed to os.environ.update() 
+                # successfully. We also need to unset anything from the environment which shouldn't be there 
+                exports = bb.data.exported_vars(the_data)
+                bb.utils.empty_environment()
+                for e, v in exports:
+                    os.environ[e] = v
 
                 if quieterrors:
                     the_data.setVarFlag(taskname, "quieterrors", "1")