default: execute 'init' after the 'location' option is automatically set

This makes 'init' able to set other options that depend on location.
It's still free to change options['location'] if the part is not shared.

slapos/recipe/build/__init__.py

@@ -226,23 +226,24 @@ class Script(EnvironMixin):
     else:
       self.keep_on_error = False
     EnvironMixin.__init__(self, False)
-    if self._init:
-      self._exec(self._init)
     shared = Shared(buildout, name, options)
     if self._update:
       shared.assertNotShared("option 'update' can't be set")
     if self._install:
-      location = options.get('location')
-      if location:
-        shared.assertNotShared("option 'location' can't be set")
-        shared.location = location
-      else:
+      if not options.get('location'):
         options['location'] = shared.location
+      if self._init:
+        self._exec(self._init)
+      if shared.location != options['location']:
+        shared.assertNotShared("option 'location' can't be set")
+        shared.location = options['location']
       shared.keep_on_error = True
       shared.mkdir_location = False
       self._shared = shared
     else:
       shared.assertNotShared("option 'install' must be set")
+      if self._init:
+        self._exec(self._init)
 
   def _exec(self, script):
     options = self.options
@@ -251,7 +252,6 @@ class Script(EnvironMixin):
       g['location'] = options['location']
     except KeyError:
       pass
-
     linecache.cache[self.name] = (
         len(script),
         None,