You can't change sys.databases. It's a system view.
log_reuse_wait_desc tells you what the log is waiting for to be reused. Nothing more. It's a report of database information, not a configuration option.
If you are in simple recovery mode, the log WILL truncate on checkpoint.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability