Remont and change a built image

[fabiodelorenzo]

<path-to-your-build-dir>/host/linux-x86/bin/simg2img system.img temp.img
mkdir system_root
sudo mount -t ext4 -o loop temp.img system_root

At this point you can make whatever changes you'd like to the files in system_root, i.e. adding apks to system/app etc...

When you're done just go back down to the output dir and do:

sudo umount system_root
<path-to-your-build-dir>/host/linux-x86/bin/img2simg temp.img system.img

You can now flash system.img using fastboot as usual.