Here's my take on the subject using only busybox commands in an ash shell. Combine all of the above with some code golf magic.

#!/bin/sh
P=/usr/lib/opkg/status
T=$(awk -v RS= '/^P.{0,8}ker/' $P|grep '^I'|cut -d' ' -f2)
awk -v RS= -v t="$T" '$0 !~t' $P|grep '^Pa'|cut -d' ' -f2|sort

But wait, can't more can be done? You are looking to list packages required for imagebuilder, and you only need the packages that automatically install all the other packages as dependencies. Fret not, there's a script for that too.

#!/bin/sh
P=/usr/lib/opkg/status
T=$(awk -v RS= '/^P.{0,8}ker/' $P|grep '^I'|cut -d' ' -f2)
I=$(awk -v RS= -v t="$T" '$0 !~t' $P|grep '^Pa'|cut -d' ' -f2|sort)
D=$(awk -v RS= -v t="$T" '$0 !~t' $P|grep '^D'|cut -d' ' -f2-|sed 's#, #\n#g'|sort|uniq)
p=;for i in $I;do if [ $(echo $D|grep -c $i) = 0 ];then p="$p $i";fi;done;echo $p

*drops mic*

Here's my take on the subject using only busybox commands in an ash shell. Combine all of the above with some code golf magic.
But wait, can't more can be done? You are looking to list packages required for imagebuilder, and you only need the packages that automatically install all the other packages as dependencies. Fret not, there's a script for that too.

#!/bin/sh
P=/usr/lib/opkg/status
T=$(awk -v RS= '/^P.{0,8}ker/' $P|grep '^I'|cut -d' ' -f2)
I=$(awk -v RS= -v t="$T" '$0 !~t' $P|grep '^Pa'|cut -d' ' -f2|sort)
D=$(awk -v RS= -v t="$T" '$0 !~t' $P|grep '^D'|cut -d' ' -f2-|sed 's#, #\n#g'|sort|uniq)
p=;for i in $I;do if [ $(echo $D|grep -c $i) = 0 ];then p="$p $i";fi;done;echo $p

This script could find only those packages that weren't included in image already (all preinstalled packages have same time), so it's pretty useless for image builder in this state.

Slightly modified script from @tinxx would give all packages required for image builder:

while read -r line; do
  case "$line" in
    "Package: "*)
        package="$line"
        _skip=false;;
    "Auto-Installed: yes")
        _skip=true;;
    "") # Empty line delimenates individual package blocks
        [ "$_skip" = true ] || echo "$package" | awk '{print $2}';;
  esac
done < /usr/lib/opkg/status | sort

@crzsotona
The title and filename of the OPs original script says "list user installed packages". That is what the script does. It does NOT list packages in the OG or custom firmware that has already been flashed to the device. Happy for you that you managed to discover that it is useless for anything else.