[broadcom]: Check for broadcom platform sdk init ready during syncd start. (#2931)

- What I did
During boot/reload time, wait in a loop to check for bcm initialization.
Break the loop, once sdk is ready to process the 'bcmcmd' request (or) loop count reached the maximum value.

- How I did it
In the existing implementation during syncd start process will sleep for a fixed time (3 secs)
for sdk initialization to happen. But the time taken for sdk initialization is varying for different platforms.
To fix this issue, the syncd start process wait in a loop and check whether sdk is ready to process 'bcmcmd' command.

- How to verify it
Check for syncd process status and interface status.
Check for syslogs and no failures related to syncd should be present.

platform/broadcom/docker-syncd-brcm/start.sh

@@ -15,7 +15,20 @@ wait_syncd() {
     done
 
     # wait until bcm sdk is ready to get a request
-    sleep 3
+    counter=0
+    while true; do
+        /usr/bin/bcmcmd -t 1 "show unit" | grep BCM >/dev/null 2>&1
+        rv=$?
+        if [ $rv -eq 0 ]; then
+            break
+        fi
+        counter=$((counter+1))
+        if [ $counter -ge 60 ]; then
+            echo "syncd is not ready to take commands after $counter re-tries; Exiting!"
+            break
+        fi
+        sleep 1
+    done
 }