This address 2 needs after the IMAGE_ROOTFS_SIZE is cleaned up, by
removing the _ext2/3 overrides it allows for a cleaner override
using IMAGE_ROOTFS_SIZE to create a large rootfs, or by setting the
IMAGE_ROOTFS_EXTRA_SPACE, will allow for extra space allocated in Kilobytes
above the base size (determined by du of the rootfs * IMAGE_OVERHEAD_FACTOR,
default of 1.3).
