Kconfig: add generated Kconfig_vlans.in for VLANs

Keep all VLAN related configuration in the Kconfig_vlans.in.
Source this file in the Kconfig
Also copy it to the target.
Kconfig_vlans.in is generated by a command:
userspace/host_tools/gen_vlan_kconf.sh > Kconfig_vlans.in

Signed-off-by: Adam Wujek <adam.wujek@cern.ch>