cmake: set.cmake: fix qi_persistent_{pre,ap}pend_uniq functions
cmake: set.cmake: fix qi_persistent_{pre,ap}pend_uniq functions

Because qi_persistent_prepend_uniq and qi_perssistent_append_uniq are
functions, they must store their result in their parent scope;
otherwise, the result is lost as soon as the function is exited.

Change-Id: I6fd107b4d8ba7e6edec931e31f28218839ab2c1a
Reviewed-on: http://gerrit.aldebaran.lan:8080/9748
Reviewed-by: cgestes <ddb78fe00abb42f91254c16a0860e38eb0781789@aldebaran-robotics.com>
Reviewed-by: smartin <edaf33f3e53483e687d304aa81a03bb5a17b9ddf@aldebaran-robotics.com>
Tested-by: smartin <edaf33f3e53483e687d304aa81a03bb5a17b9ddf@aldebaran-robotics.com>

aldebaran/qibuild,dmerejkowsky/qibuild,aldebaran/qibuild,dmerejkowsky/qibuild,dmerejkowsky/qibuild,dmerejkowsky/qibuild,dmerejkowsky/qibuild,aldebaran/qibuild,aldebaran/qibuild