Make sure unpack_bootimg is build
cd ~/android/twrp
source build/envsetup.sh
# For the Atom L EEA use
breakfast Atom_L_EEA
# For the atom XL EEA use
breakfast Atom_XL_EEA
# For the Atom L TEE use
breakfast Atom_L_TEE
# For the atom XL TEE use
breakfast Atom_XL_TEE
make unpack_bootimg
unpack_bootimg will be built in
~/android/twrp/out/host/linux-x86/bin/unpack_bootimg
Head on over to xda-developers.com to download a complete OTA tools package and extract it to a folder of your choice.
Download and install imjtool (formerly known as imgtool)
mkdir -p ~/bin
wget http://newandroidbook.com/tools/imjtool.tgz
tar -xzf imjtool.tgz -C ~/bin
Use unpack_bootimg extract all required infos from boot.img and recovery.img of the stock rom
./unpack_bootimg --boot_img boot.img --out boot
./unpack_bootimg --boot_img recovery.img --out recovery
You will get all informations required for BoardConfig.mk and a kernel, a dtb and a dtbo file (only from recovery).
Download the latest ROM from Unihertz' Google Drive. Because these archives are different from release to release I cannot tell you exactly where to find them and how the are structured. You just need to extract the file super.img and put it into a folder where you could easily find it. For this example I put it into ~/unihertz.
Now we need to extract the different images from the super.img
cd ~/unihertz
rm -rf extracted/
imjtool super.img extract
imjtool extracted/image.img extract
Next we need to mount them into the filesystem
cd ~/unihertz/extracted
mkdir -p system/
sudo mount -o loop system.img system/
sudo mount -o loop vendor.img system/vendor/
sudo mount -o loop product.img system/product
Now you can browse your phones filesystem and extract everything you need
To cleanup just unmout the images
sudo umount ~/unihertz/extracted/system/product
sudo umount ~/unihertz/extracted/system/vendor
sudo umount ~/unihertz/extracted/system