fs/ext2: Add BR2_TARGET_ROOTFS_EXT2_MKFS_OPTIONS option

This option lets the user specify mke2fs options and/or ext2/3/4
features.

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>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
[Thomas: improve wording a bit, fix typos.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>