support/scripts/scancpan: automatically populate LICENSE_FILES

This commit improves the scancpan script to automatically populate the
LICENSE_FILES variable using informations available in the Perl
package MANIFEST file.

Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>