Sort the results of findleaves again.

Commit d36e945e changed findleaves.py to sort the output files so that
the order is deterministic and won't cause spurious build errors when
files happen to be listed (and hence read) in different order on
e.g. add/remove the makefiles.

This correction broke after commit 4820a945 because the conversion to
a set with set() in order to remove duplicates came after sort() and
potentially altered the order of the elements.

This commit fixes makes sure findleaves.py is guaranteed to return
a sorted filename list without duplicates.

Change-Id: Ifd96d04d45641fe43d4cc4739f78a2d4d25cc212