Hello
Please have a look at the update.sh script.
The partitioning is done here: update script mbr create
This creates an MBR with the boot and rootfs partitions. Reduce the size of the rootfs partition here and create your third one.
Now, after running update.sh and updating your module you have 3 partitions, the last one waiting for you to create a filesystem on it.
If you want to create and deploy the filesystem also on the development PC you could do something like this for the filesystem on the third partition: update script fs create
And adding it also to the flashing U-Boot script:
U-Boot script
Note that we create the rootfs smaller than the partition it lives in. The following adds a systemd task and script to expand the file system to the full size of the partition it lives in. recipe
One probably could change /etc/fstab to have /dev/mmcblk0p1 mounted read only or not mounted at all.
I have no experience for having a read only rootfs.
Max