[config-chassisdb] use cached variables (sonic-net#17342)

- 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 <stepanb@nvidia.com>

files/image_config/config-chassisdb/config-chassisdb

@@ -27,7 +27,7 @@
 config_chassis_db() {
     startdb_file="/etc/sonic/chassisdb.conf"
     [ ! -e $startdb_file ] || rm $startdb_file
-    platform=$(sonic-cfggen -H -v DEVICE_METADATA.localhost.platform)
+    platform=${PLATFORM:-`sonic-cfggen -H -v DEVICE_METADATA.localhost.platform`}
     # database-chassis services will start when $chassis_config file exists
     chassis_config="/usr/share/sonic/device/$platform/chassisdb.conf"
     if [ ! -e $chassis_config ]; then
@@ -54,6 +54,9 @@ config_chassis_db() {
     fi
 }
 
+# read SONiC immutable variables
+[ -f /etc/sonic/sonic-environment ] && . /etc/sonic/sonic-environment
+
 config_chassis_db
 
 exit 0