If we don't do this, multilib and other varients using BBCLASSEXTEND
will fail.
(this patch is a subset of 9a97367038a1e2431bf94211dabbc5aedbbee3bb - Ross Burton)
(From OE-Core rev: da81f2816086c240ad5ebf74f3b8a2caed0b6cae)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>