wic: use bitbake variable ROOTFS_SIZE

If bitbake image is referenced in .ks file and --size is not used
there wic uses ROOTFS_SIZE variable to set minimum partition size.

ROOTFS_SIZE is calculated in meta/lib/oe/image.py when rootfs is
created. The calculation is done using other image parameters:
IMAGE_ROOTFS_SIZE, IMAGE_ROOTFS_ALIGNMENT, IMAGE_OVERHEAD_FACTOR
and IMAGE_ROOTFS_EXTRA_SPACE.

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>