lantiq: split up dsl_notify.sh into separate notification handlers

lantiq: split up dsl_notify.sh into separate notification handlers

This makes it possible to simply add additional notification handlers
by putting new scripts in the /etc/hotplug.d/dsl directory.

the following Environment Variables are used:

(values marked with an * are only available when bonding support is
compiled in)

DSL_NOTIFICATION_TYPE with one of these values:
- DSL_STATUS
- DSL_INTERFACE_STATUS
- DSL_DATARATE_STATUS_US
- DSL_DATARATE_STATUS_DS

DSL_LINE_NUMBER=<0|1> *

If DSL_NOTIFICATION_TYPE == DSL_STATUS, these variables are set:

- DSL_XTU_STATUS=<ADSL|VDSL>
- DSL_TC_LAYER_STATUS=<ATM|EFM>
- DSL_EFM_TC_CONFIG_US=<NORMAL|PRE_EMPTION>
- DSL_EFM_TC_CONFIG_DS=<NORMAL>

If DSL_NOTIFICATION_TYPE == DSL_INTERFACE_STATUS, these variables are
set:

- DSL_INTERFACE_STATUS=<DOWN|READY|HANDSHAKE|TRAINING|UP>
- DSL_BONDING_STATUS=<INACTIVE|ACTIVE> *

If DSL_NOTIFICATION_TYPE == DSL_DATARATE_STATUS_US, these variables are
set:

- DSL_DATARATE_US_BC0=<Upstream data rate in bit/s for Channel 0>
- DSL_DATARATE_US_BC1=<Upstream data rate in bit/s for Channel 1> *

If DSL_NOTIFICATION_TYPE == DSL_DATARATE_STATUS_DS, these variables are
set:

- DSL_DATARATE_DS_BC0=<Downstream data rate in bit/s for Channel 0>
- DSL_DATARATE_DS_BC1=<Downstream data rate in bit/s for Channel 1> *

Signed-off-by: Martin Schiller <ms@dev.tdt.de>