Better handling of clean

Summary:
make clobber used to call rm -rf out/
so if out is a soft link to i.e. /tmp/out
the soft link is deleted instead the content
so i changed the makefile to rm -rf out/*

Change-Id: Id1991161617da8ec34307a30412c134e31f589b4

Test Plan: None.

Reviewers: randall.rushing, Mustaavalkosta, rlasalle15, michyprima, Roach, synergy

Reviewed By: Mustaavalkosta

Differential Revision: http://review.codefirex.com/D216