MLBEDSW-2596 Add support to set driver log severity

The driver can log at one of the following levels: 0=emerg,
1=alert, 2=crit, 3=err, 4=warning, 5=notice, 6=info, 7=debug.

The logs at or below the set level are printed whereas
the higher level logs are not printed.

Change-Id: I05a498d4c8c78112207187d9dceaa2386b138c5d