Makefile: add pkg-stats target

Now that pkg-stats is not just a maintainer-oriented tool, but a tool
generally useful to users, introduce a make target to run
pkg-stats. Of course, it is run with the newly introduced -c option,
which produces a pkg-stats output for just the selection of packages
of the currently defined configuration.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>