support/scripts/pkg-stats: improve 'package_init_make_info'

support/scripts/pkg-stats: improve 'package_init_make_info'

The pkg-stats calls 3 times `make` to get a bunch of variables. These
variables can be obtained in only one make invocation.  This patch
replaces the three calls by just one and adjusts the parsing logic
accordingly.

Note: another option suggested by Arnout would be to run `make
show-info` that produces a json with the necessary variables.  This
would avoid the duplicated effort done in pkg-stats and pkg-utils and
allow to add other infos to pkg-stats like dependencies, reversed
dependencies or if the package is virtual.

In order to use this method, the following changes are required in
pkg-generic's show-info:

 - include license_files;
 - have an option to run it on *all* packages, not just the selected
   ones.

This patch take the simplest approach of only factorizing the make
calls as it requires less changes.

Signed-off-by: Victor Huesca <victor.huesca@bootlin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>