hwsim tests: Move logging into timestamped subdirectory

hwsim tests: Move logging into timestamped subdirectory

Instead of logging many files called "<timestamp>-*", log into
a new "<timestamp>/*" in the logs/ subdirectory and only put
the last-debug file into logs/. If a LOGDIR is specified in the
environment, instead just put everything into that directory
(so the caller should make sure to give it a timestamp or so)
and skip the creation of last-debug entirely.

Also clean up a bit and pass the LOGDIR from run-all.sh to
start.sh rather than having start.sh create the timestamp and
run-all.sh detect the latest one when having run start.sh.

Signed-hostap: Johannes Berg <johannes.berg@intel.com>