rootfs-postcommands: fix symlinks where link and output path are equal

When creating the manifest and the testdata.json links, if the link
name is equal to the output name the link is not created, otherwise
it is. This prevents a link-to-self in the first case.

Signed-off-by: Davide Gardenal <davide.gardenal@huawei.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>