Makefile: targets are now declared phony by the appropriate infrastructures

The main Makefile was declaring a subset of the per-package targets as
being PHONY, but not all of them. Now that the pkg-generic package
infrastructure is taking care of that in a much more systematic
fashion, this commit gets rid of the unneeded code from the main
Makefile.

[Thomas: re-add list-defconfigs to the list of PHONY targets, as
noticed by Yann.]

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