[ntp] disable ntp time jump (sonic-net#2589)

- removing -g to disable jump when time difference is greater than 1000s
- add -x to disable initial jump

build_debian.sh

@@ -263,6 +263,10 @@ sudo mv $FILESYSTEM_ROOT/grub-pc-bin*.deb $FILESYSTEM_ROOT/$PLATFORM_DIR/x86_64-
 ## Disable kexec supported reboot which was installed by default
 sudo sed -i 's/LOAD_KEXEC=true/LOAD_KEXEC=false/' $FILESYSTEM_ROOT/etc/default/kexec
 
+## Modifty ntp default configuration: disable initial jump (add -x), and disable
+## jump when time difference is greater than 1000 seconds (remove -g).
+sudo sed -i "s/NTPD_OPTS='-g'/NTPD_OPTS='-x'/" $FILESYSTEM_ROOT/etc/default/ntp
+
 ## Fix ping tools permission so non root user can directly use them
 ## Note: this is a workaround since aufs doesn't support extended attributes
 ## Ref: https://github.com/moby/moby/issues/5650#issuecomment-303499489