ltq-vdsl-app: make it possible to configure ADSL/VDSL independently

There are some cases where ISPs are running ATM over VDSL or PTM over
ADSL, this is not the common case, but these cases exist. Make it
possible to configure OpenWrt for such cases by adding a new config
option line_mode.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

SVN-Revision: 48947