Make vlan_file optional if dynamic_vlan is used

Make vlan_file optional if dynamic_vlan is used

My APs generate their configuration on their own using a different
number of (vlan-enabled) bss. Currently, all my vlan_file files consist
of a single line: the wildcard line. Configuration file generation would
be easier, if the hostapd configuration file would not depend on those
simple vlan_file files.

This patch removes the need for those one-line files by using the
<device>.<vlan> naming scheme if no vlan_file is given (or that file is
empty). This should not break any existing setup, as using dynamic_vlan
with no vlan configured does not make sense anyway.

Signed-hostap: Michael Braun <michael-dev@fami-braun.de>