[config-topology] use cached variables (#17343)

- Why I did it
Improve  boot performance mostly needed for fast and warmboot

- How I did it
Use cached variable.

- How to verify it
Boot the system. Simply do "systemd-analyze blame" and look at service start time.

Signed-off-by: Stepan Blyschak <[email protected]>

files/image_config/config-topology/config-topology.sh

@@ -4,7 +4,7 @@
 # which could be used for platform specific topology configuration
 #
 start() {
-    PLATFORM=`sonic-cfggen -H -v DEVICE_METADATA.localhost.platform`
+    PLATFORM=${PLATFORM:-`sonic-cfggen -H -v DEVICE_METADATA.localhost.platform`}
     #Path to platform topology script
     TOPOLOGY_SCRIPT="/usr/share/sonic/device/$PLATFORM/plugins/config-topology.sh"
     #if topology script file not present, do nothing and return 0
@@ -13,7 +13,7 @@ start() {
 }
 
 stop() {
-    PLATFORM=`sonic-cfggen -H -v DEVICE_METADATA.localhost.platform`
+    PLATFORM=${PLATFORM:-`sonic-cfggen -H -v DEVICE_METADATA.localhost.platform`}
     #Path to platform topology script
     TOPOLOGY_SCRIPT="/usr/share/sonic/device/$PLATFORM/plugins/config-topology.sh"
     #if topology script file not present, do nothing and return 0