base-files: uci-defaults-new, config_generate: refactor code and drop old vlans

Remove support for now unused ucidef_add_switch_vlan(), move port->vlan and
vlan->interfaces conversion to uci-defaults-new.sh and massively simplify
config_generate.

This change prepares the following upcoming steps:

 * Eliminate use of ucidef_set_interface_lan_wan() for switch only devices
 * Merge ucidef_add_switch() with ucidef_add_switch_ports()

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

SVN-Revision: 47719