support/scripts: add size-stats script

This new script uses the data collected by the step_pkg_size
instrumentation hook to generate a pie chart of the size contribution
of each package to the target root filesystem, and two CSV files with
statistics about the package size and file size. To achieve this, it
looks at each file in $(TARGET_DIR), and using the
packages-file-list.txt information collected by the step_pkg_size
hook, it determines to which package the file belongs. It is therefore
able to give the size installed by each package.

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