In case someone is anxious for this, here is a quick and dirty guide how to do it:
1.) Grab my stock system.img.ext4 from stock odin post
2.) Boot into a linux with ext4 support compiled into kernel and e2fsprogs installed
3.) Put the system.img.ext4 in your home folder
(from your home folder)
4.) sudo mkdir /mnt/system
5.) sudo mount system.img.ext4 /mnt/system
6.) look in /mnt/system, here is the whole filesystem mounted as a folder. you can add/remove/change things at will from here, like adding su to /mnt/system/xbin, theme stuff, whatever. every change is preserved in the system.img.ext4.
7.) leave the /mnt/system folder so it's not in use
8.) df -h (note the size of /mnt/system in MB)
9.) sudo umount /mnt/system
10.) sudo e2fsck system.img.ext4 (say yes to the prompt asking to create lost+found so it is marked clean)
11.) sudo resize2fs system.img.ext4 790M (this is assuming df -h read 785MB, I am leaving 5MB free space. You can make this any size up to 1GB which is the full system partition size)
12.) (optional) re-mount system.img.ext and delete lost+found folder, then umount it. Android doesn't require that folder, we just needed e2fsck to mark the filesystem clean before resize2fs resized it.
(you should now have a system.img.ext4 at the size you set in resize2fs step.)
13.) tar cvpf pda-odin-system.tar system.img.ext4
That's it, you've cooked up a custom system.img.ext4 to be flashed by Odin to /system, this is my favorite way to mod a ROM
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