lib/oe/rootfs: prelink only when image-prelink is inherited

lib/oe/rootfs: prelink only when image-prelink is inherited

Use prelink only when image-prelink class is inherited. Inheriting this
class adds prelink-native as dependency to do_rootfs, which makes prelink
binary and configuration available.

|Exception: FileNotFoundError: [Errno 2] No such file or directory:
|'/home/.../1.0-r0/recipe-sysroot-native/etc/prelink.conf'
|
| if not os.path.exists(prelink_cfg):
|     shutil.copy(self.d.expand('${STAGING_DIR_NATIVE}${sysconfdir_native}/prelink.conf'),prelink_cfg)

(From OE-Core rev: 9e30e72920b3c661279499f2e94d5dc0087a1a8e)

Signed-off-by: Andrej Valek <andrej.valek@siemens.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>