package/mke2img: add option to specify block size

package/mke2img: add option to specify block size

ext filesystems can use a block size of 1024, 2048, or 4096 bytes, the
former being interesting to store small files, while the latter being
more intersting to store bigger files.

So far, we were using the default, which was to use a 1024-byte block.
Continue doing so (for now...).

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>