In recovery mode, recovery may update /boot or /system, but it never overwrite itself. The update of /recovery is in the normal bootup. When system boot up, it will execute init.rc which will call install-recovery.sh.
The install-recovery.sh is in update.zip. when the system is in recovery mode, updater-script will unzip update.zip, and the install-recovery.sh will be unzip into /system/etc/. So if you update your image through recovery mode, the install-recovery.sh will be unzip to /system/etc/ automatically.
If your update.zip do not include install-recovery.sh. You can edit it and copy it to /system/etc. the below is content in install-recovery.sh.
if ! applypatch -c EMMC:/dev/block/mmcblk3p2:7762488:374c3807940a38d9497a4c5ef64a069e553bc218; then