Before this change, running prune-trace with, e.g.
"prune-trace -d fsp_mibench -v bitmap% --benchmark-exclude clockcnv"
resulted in an implied "--benchmark none", rendering --benchmark-exclude
ineffective and resulting in nothing being pruned. Now, the "none" default
only applies when neither --benchmark nor --benchmark-exclude (analogously
for --variant / --variant-exclude) is provided.
Change-Id: Ic7c88919d7cfde1261749a745dc6a679472ff348
36ae6fd6c3