Add sparse ext4 tools, zerofree and sparsify

Reference: http://intgat.tigress.co.uk/rmy/uml/index.html

As an example, Galaxy Tab 3 T31x with dual boot support enabled
using sparse ext4 system.img in ext4 /data partition, we can
reclaim deleted blocks and make it sparse like so:

    mount /data
    zerofree /data/media/.secondrom/system.img
    umount /data
    sparsify /dev/block/platform/dw_mmc/by-name/USERDATA \
    /media/.secondrom/system.img