pkg-generic: declare phony targets as such

This commit improves the pkg-generic package infrastructure so that it
declares all the <pkg>-<something> targets as PHONY, which they
actually are.

[Thomas:
   - add some missing phony targets, noticed by Yann E. Morin;
   - put one target per line, order alphabetically. Makes it easier to
     spot missing targets.]

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>