Increase sparse image block size when ROOTFS_SIZE is smaller than
the minimum needed for ext4 to fit into it.
[YOCTO #7664]
(From OE-Core rev: 6938791ff97a23430afb4aa16d71aa8729a12ead)
Signed-off-by: Alex Franco <alejandro.franco@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>