[fast-reboot] Fix regression: set FAST_REBOOT state_db flag to support fast-reboot from older images (#16733)

Why I did it
Fix: #16699

Fast reboot is failing from old OS versions (eg., 201911 image) to latest (eg., master branch) after PR #15685

The system wide flag for FAST_REBOOT is still required when the base OS version does not support the new fast-reboot reconciliation logic (no db dump)

Author: vaibhavhd

files/build_templates/docker_image_ctl.j2

@@ -247,6 +247,12 @@ function postStartAction()
                     $SONIC_CFGGEN -j /etc/sonic/config_db$DEV.json --write-to-db
                 fi
             fi
+
+            if [[ "$BOOT_TYPE" == "fast" ]]; then
+                # this is the case when base OS version does not support fast-reboot with reconciliation logic (dump.rdb is absent)
+                # In this case, we need to set the flag to indicate fast-reboot is in progress. Set the key to expire in 3 minutes
+                $SONIC_DB_CLI STATE_DB SET "FAST_REBOOT|system" "1" "EX" "180"
+            fi
         fi
 
         if [ -e /tmp/pending_config_migration ] || [ -e /tmp/pending_config_initialization ]; then