fs/common: always depend on build host-tar if needed

fs/common: always depend on build host-tar if needed

Currently, the filesystems do not depend on building host-tar when it
is needed, even though all of them have to extract the intermediate
tarball.

However, in degenerate (but legally valid) configurations with no
user-selectable package selected, host-tar would not be built, so the
rootfs images would use whatever improper tar the system has.

Add the conditional dependency to host-tar to the rootfs-common
intermediate image. Since this is the internal step that all real rootfs
generators depend on, they now properly depend on host-tar when needed.

In practice, when host-tar is needed, it will always be built before the
rootfs images, because it is a dependency of all packages (except a very
few, like the skeleton), of which host-fakeroot, which is a mandatory
dependency of rootfs-comon anyway. But for consistency sake, let's
explicitly add host-tar as a dependency to rootfs-common too.

Note that rootfs-tar already had that dependency, and we leave it as-is
because it is semantically correct, even if superfluous.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>