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" <[email protected]>
Cc: Thomas Petazzoni <[email protected]>
Cc: Arnout Vandecappelle <[email protected]>
Reviewed-by: Romain Naour <[email protected]>
Signed-off-by: Thomas Petazzoni <[email protected]>