add basic interface alias support.

add basic interface alias support.
for now (since we don't use ip in our network scripts yet), it adds :1, :2, ... alias interfaces
this will change when we add support for the ip utility, but the config format will remain the same.
example:

config alias     test
option interface lan
option ipaddr    192.168.1.1
option netmask   255.255.0.0

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@12241 3c298f89-4303-0410-b956-a3cf2f4a3e73