fs/tar: use qstrip to clean up build output

When using the tar filesystem method, the build output shows at the end:

echo " tar -c""f <br>/output/images/rootfs.tar -C <br>/output/target ." >>
                                    <br>/output/build/_fakeroot.fs

The inner set of quotes can be easily removed by properly stripping the tar
options (coming from the buildroot configuration).

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>