**- Why I did it**
PR https://github.com/Azure/sonic-buildimage/pull/4599 introduced two bugs in the startup of the router advertiser container:
1. References to the `wait_for_intf.sh` script were changed to `wait_for_link.sh`, but the actual script was not renamed
2. The `ipv6_found` Jinja2 variable added to the supervisor config file goes out of scope before it is read.
**- How I did it**
1. Rename the `wait_for_intf.sh` script to `wait_for_link.sh`
2. Use the Jinja2 "namespace" construct to fix the scope issue
**- How to verify it**
Ensure all processes in the radv container start properly under the correct conditions (i.e., whether or not there is at least one VLAN with an IPv6 address assigned).
Calls to sonic-cfggen is CPU expensive. This PR reduces calls to
sonic-cfggen to one call during startup when starting radv service.
singed-off-by: Tamer Ahmed <tamer.ahmed@microsoft.com>
