Here u go. My bootimage extract and pack package. It only works on linux. See the zip, and extract it somewhere in your linux distribution.
1. Put a boot.img in the root of the extracted bootimage packages (where the folders Ramdisk, Bootimage, Kernel, Tools are)
2. Execute the file extract.
3. Kernel can be found under the map kernel
4. Ramdisk can be found under the map Ramdisk/ramdisk
5. Modify your things
6. Pack the files by executing create.
In the map Bootimage is a bootstub (bootold.img) needed for rebuilding the boot.img and some old scripts
In the map Kernel will be the extracted kernel. The file bzImage will always be used for rebuilding.
In the map Ramdisk will be the extracted ramdisk in the map ramdisk. The map 'ramdisk' will always be used for rebuilding,
In the map Tools will be the tools needed for extracting and creating.
If u have some questions about this tool, ask!
Credits to Turl1 for the tools