slapos.recipe.build: restore support for build scripts

This fixes this error:
[2015-04-10 06:28:09,906] INFO         exec self.options['script']
[2015-04-10 06:28:09,906] INFO       File "<string>", line 6, in <module>
[2015-04-10 06:28:09,906] INFO     NameError: name 'env' is not defined

When installing busybox

slapos/recipe/build/__init__.py

@@ -277,6 +277,7 @@ class Script:
       self.keep_on_error = True
     else:
       self.keep_on_error = False
+
   def getEnvironment(self):
     # prepare cool dictionary
     wanted_env = {}
@@ -313,6 +314,7 @@ class Script:
         self.cleanup_dir_list.append(self.options['location'])
         raise shutil.Error('Directory %s already exists' % 
             self.options['location'])
+      env = self.getEnvironment() # env is used in script exec'ed below
       exec self.options['script']
       try:
         self._checkPromise('slapos_promise', self.options['location'])
@@ -342,6 +344,7 @@ class Script:
     if self.update_script is None:
       return None
     try:
+      env = self.getEnvironment() # env is used in script exec'ed below
       exec self.options['update_script']
       try:
         self._checkPromise('slapos_update_promise', self.options['location'])