[syncd_init_common.sh] fix fast reboot backwards compatibility (sonic-net#480)

* [syncd_init_common.sh] fix fast reboot backwards compatibility
We should handle both cases for backward-compatible with 201803:
 - fast-reboot
 - SONIC_BOOT_TYPE=fast-reboot
* handle review comments

Author: stepanblyschak

syncd/scripts/syncd_init_common.sh

@@ -24,17 +24,15 @@ fi
 # Use temporary view between init and apply
 CMD_ARGS+=" -u"
 
-BOOT_TYPE="$(cat /proc/cmdline | grep -o 'SONIC_BOOT_TYPE=\S*' | cut -d'=' -f2)"
-
-case "$BOOT_TYPE" in
-  fast-reboot)
-     FAST_REBOOT='yes'
-    ;;
-  fastfast)
+case "$(cat /proc/cmdline)" in
+  *SONIC_BOOT_TYPE=fastfast*)
     if [ -e /var/warmboot/warm-starting ]; then
         FASTFAST_REBOOT='yes'
     fi
     ;;
+  *SONIC_BOOT_TYPE=fast*|fast-reboot*)
+     FAST_REBOOT='yes'
+    ;;
   *)
      FAST_REBOOT='no'
      FASTFAST_REBOOT='no'