util: support "y" and "n" in env_var_as_boolean()

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>