generate_descriptor_pkg.sh: improved detection of allowed files

This change introduces a new check for other allowed files, as long
as its name is defined in ALLOWED_FILES_WILDCARDS. For instance, with
this change, README can now be README.txt, README.md, README.rst, etc.

Change-Id: Ia9a0bfb455b08fd5a8a924508f5611de57754454
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>