fs/ext2: rename BR2_TARGET_ROOTFS_EXT2_BLOCKS to BR2_TARGET_ROOTFS_EXT2_SIZE

This change deprecates the ext2/3/4 rootfs size in blocks symbol in
favor of one that mimic the fs-size argument behavior of mkfs (i.e.
size in a human readable format accepting k, m, g or t suffix or their
upper-case variants).

This change also updates the defconfigs that used to set
BR2_TARGET_ROOTFS_EXT2_BLOCKS symbol.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>