fs/ext2: deprecate BR2_TARGET_ROOTFS_EXT2_EXTRA_INODES option

fs/ext2: deprecate BR2_TARGET_ROOTFS_EXT2_EXTRA_INODES option

Soon, the ext{2,3,4} rootfs image will no longer be generated with
genext2fs, but using mke2fs instead which has no support for the extra
inode number option.

So, deprecate the BR2_TARGET_ROOTFS_EXT2_EXTRA_INODES option and
recommend, in lieu, to set the total inode number, taking account of
the extra ones if needed.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Cc: Sébastien Szymanski <sebastien.szymanski@armadeus.com>
Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>