Improve logic to find default certificates in createpackage script

Improve logic to find default certificates in createpackage script

Original implementation assumed createpackage script was always run
from under Qt source tree bin directory, which is not always the case
as on some platforms the Qt tools can be found from under EPOCROOT.

Fixed it so that if the default directory for default certificates
can't be found in the expected location, createpackage will attempt to
query qmake in the same directory as the createpackage script for the
location of the Qt source tree (QT_INSTALL_PREFIX) and look for default
certificates directory from under there.

Task-number: QTBUG-18684
Reviewed-by: Janne Koskinen
(cherry picked from commit 01477af79d8114b3f8993c3967892538a599dfa6)