Prevent footer from being included in `rootfs.img` when extracting

[SomeInterestingUserName]

I noticed that extract_firmware.sh was duplicating the contents of the footer in fmk/image_parts/rootfs.img. As far as I'm aware, it hasn't caused any major problems in my workflow. However, I imagine it could throw off size calculations using these files since the combined size of files in image_parts would be bigger than expected by the length of the footer.

I've modified the section in the script that extracts rootfs.img to take into account the footer offset and extract only up to that point.