fs/ext2: use mkfs to generate rootfs image

fs/ext2: use mkfs to generate rootfs image

mkfs is now capable of generating rootfs images. Use mkfs instead of
genext2fs.

If not set, we now let mkfs calculate the block size and the number of
inodes needed.

This change also adjusts the options to meet those of mkfs.

Notes:
* Passing a null inode number to mkfs triggers its automatic calculation.
* Passing a fs-size with no unit suffix to mkfs is interpreted as using
  1K block size.

Signed-off-by: Sébastien Szymanski <sebastien.szymanski@armadeus.com>
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
[Thomas: don't pass PATH when calling mkfs.ext, just call it directly
from $(HOST_DIR)/usr/sbin, as suggested by Arnout.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>