[docker-radv] Fix startup issues (#5230)

**- 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).

dockers/docker-router-advertiser/Dockerfile.j2

@@ -25,7 +25,7 @@ RUN apt-get clean -y        && \
     rm -rf /debs
 
 COPY ["docker-init.sh", "/usr/bin/"]
-COPY ["radvd.conf.j2", "wait_for_intf.sh.j2", "docker-router-advertiser.supervisord.conf.j2", "/usr/share/sonic/templates/"]
+COPY ["radvd.conf.j2", "wait_for_link.sh.j2", "docker-router-advertiser.supervisord.conf.j2", "/usr/share/sonic/templates/"]
 COPY ["files/supervisor-proc-exit-listener", "/usr/bin"]
 COPY ["critical_processes", "/etc/supervisor"]
 

dockers/docker-router-advertiser/docker-init.sh

@@ -8,10 +8,10 @@ CFGGEN_PARAMS=" \
     -d \
     -t /usr/share/sonic/templates/docker-router-advertiser.supervisord.conf.j2,/etc/supervisor/conf.d/supervisord.conf \
     -t /usr/share/sonic/templates/radvd.conf.j2,/etc/radvd.conf \
-    -t /usr/share/sonic/templates/wait_for_intf.sh.j2,/usr/bin/wait_for_intf.sh \
+    -t /usr/share/sonic/templates/wait_for_link.sh.j2,/usr/bin/wait_for_link.sh \
 "
 sonic-cfggen $CFGGEN_PARAMS
 
-chmod +x /usr/bin/wait_for_intf.sh
+chmod +x /usr/bin/wait_for_link.sh
 
 exec /usr/bin/supervisord

dockers/docker-router-advertiser/docker-router-advertiser.supervisord.conf.j2

@@ -26,20 +26,21 @@ stdout_logfile=syslog
 stderr_logfile=syslog
 dependent_startup=true
 
-{# Router advertiser should only run on ToR (T0) devices #}
-{% if DEVICE_METADATA.localhost.type == "ToRRouter" %}
-
-  {% if VLAN_INTERFACE %}
-    {% for (name, prefix) in VLAN_INTERFACE|pfx_filter %}
+{# Router advertiser should only run on ToR (T0) devices which have #}
+{# at least one VLAN interface which has an IPv6 address asigned #}
+{%- set vlan_v6 = namespace(count=0) -%}
+{%- if DEVICE_METADATA.localhost.type == "ToRRouter" -%}
+  {%- if VLAN_INTERFACE -%}
+    {%- for (name, prefix) in VLAN_INTERFACE|pfx_filter -%}
       {# If this VLAN has an IPv6 address... #}
-      {% if prefix | ipv6 %}
-        {% set ipv6_found = true %}
-      {% endif %}
-    {% endfor %}
-  {% endif %}
+      {%- if prefix | ipv6 -%}
+        {%- set vlan_v6.count = vlan_v6.count + 1 -%}
+      {%- endif -%}
+    {%- endfor -%}
+  {%- endif -%}
+{%- endif -%}
 
-  {# Enusre at least one ipv6 vlan interface #}
-  {% if ipv6_found == true %}
+{%- if vlan_v6.count > 0 %}
 [program:wait_for_link]
 command=/usr/bin/wait_for_link.sh
 priority=3
@@ -60,6 +61,4 @@ stdout_logfile=syslog
 stderr_logfile=syslog
 dependent_startup=true
 dependent_startup_wait_for=wait_for_link:exited
-  {% endif %}
-
-{% endif %}
+{% endif -%}