@jonius Ok let me explain with a screenshot - this one is from the old boot layout, where android system is inside ubuntu.img and system.img is a hardlink to ubuntu.img, so please forgive the sizes of the files. This WILL change with Halium):
partition BOOT: Contains initrd system with kernel and the usual initrd helpers
0_1528024151564_5187ab83-cd25-4f04-85c8-4557a2dd807c-grafik.png
partition DATA: ext4 formatted, contains:
on /data:
ubuntu.img is the read-only root filesystem
system.img contains the Android LXC container. This will not be needed at all for Raspberry
folder user-data: Contains all user data from the home dir
folder system-data:Contains system settings like WiFi, network, future printing, etc.
So, if you manage to boot the raspberry image, your bootloader basically needs to follow the steps of https://github.com/ubports/ubports-boot and its initrd workflow. Specifically, see this init: https://github.com/ubports/ubports-boot/blob/master/initramfs/init
BR