pkg-kconfig: declare phony targets as such

Just like the previous commit did for the pkg-generic infrastructure,
this commit improves the pkg-kconfig infrastructure to declare its
custom <pkg>-<something> targets as PHONY.

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