Packing/repacking Android boot images is annoying. Android Boot Image Tools makes it a breeze.

Make sure cpio, mkbootimg, and unmkbootimg are on your $PATH.

cpio seems to be fairly standard and shouldn't require special installation. mkbootimg/unmkbootimg can be found here: https://github.com/pbatard/bootimg-tools

abit get-kernel boot.img kernel

abit get-ramdisk boot.img ramdisk

abit put-kernel boot.img kernel [-o boot2.img]

abit put-ramdisk boot.img ramdisk [-o boot2.img]