fs/ext2: use a post-gen hook rather than a post-target rule

fs/ext2: use a post-gen hook rather than a post-target rule

post-target rules are probably not resilient to parallel builds, given
that they do not depend on the image being generated first.

Beside, we already have a mechanism for running stuff after the
filesystem is generated, and that's called post-gen hooks.

Use those hooks.

Note: this basically reverts 75b6303 (rootfs-ext2: make the symlink as a
_POST_TARGET) since we've now re-introduced post-gen hooks.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>