support/scripts/pkg-stats: allow to run script outside of the top-level directory

Currently, pkg-stats expects being executed from Buildroot's top-level
source directory. As we are going to extend pkg-stats to cover only
the packages available in the current configuration, it makes sense to
be able to run it from the output directory, which can be anywhere
compared to Buildroot's top-level directory.

This commit adjusts pkg-stats to this, by inferring all Buildroot
paths based on the location of the pkg-stats script itself.

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