package/mke2img: set sparse_super for revision 1

The sparse_super feature is more efficient in terms of speed and
available space, so set it by default if supported, i.e. for revision 1.

This gives a huge speed gain for large resize2fs operations, especially
for expansion.

Signed-off-by: Benoît Thébaudeau <benoit@wsystem.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>