package/pkg-generic: mark some rule PHONY

package/pkg-generic: mark some rule PHONY

The %-graph-rdepends should be marked PHONY, as the %-graph-depends is.

The %-show-dependency-tree should also be a PHONY rule, because it does
not actually create a file. Furthermore, to avoid the "Nothing to be
done for .." message, give that rule an actual recipe that just does
nothing.

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>