Add FORCE_REBUILD parameter
Add FORCE_REBUILD parameter

Instead of manually removing the two *.decscription files forces the
image to be rebuild when calling it with the FORCE_REBUILD parameter,
for example:

 FORCE_REBUILD=1 ./run_functional-tests.sh

Change-Id: Ieee06e8eed791f23e1893f337ffcfd76c4e17edd

enovance/software-factory,enovance/software-factory,enovance/software-factory,enovance/software-factory,enovance/software-factory