Image type specific size overrides for IMAGE_ROOTFS were being ignored.
Moving the size calculation to after the overide expansion point addresses
this problem.
[YOCTO #928]
(From OE-Core rev: 2f676dadebebc461c1e01e0b6da3cb2258829048)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>