Finally you have to rebuild the image, to do this first compress the new image file with zip:
zip -r update-unsigned.img *
And then you have to sign that update-unsigned.img
java -Xmx1024m -jar signapk.jar -w testkey.x509.pem \ testkey.pk8 update-unsigned.img update.img
I extracted the signapk.jar, testkey.x509 and testkey.pk8 from the buildroot.
Have fun with this!
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