makebs: fix config crash

makebs: fix config crash

Without this, running makebuildserver from a clean master results in the
following:

	 $ ./makebuildserver
	Traceback (most recent call last):
	  File "./makebuildserver", line 74, in <module>
	    del(config['__builtins__'])  # added by compile/exec
	KeyError: '__builtins__'

This is because a clean checkout has no config, thus exec is never
actually ran.