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

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>
(cherry picked from commit bed63756c56f296ff3d5a7eef66e978bd19f1008)
Signed-off-by: Steve Sakoman <steve@sakoman.com>