There is an excellent post on unpacking and repacking the boot.img for Android systems, however, not only the the links to scripts are broken, also they may not work for certain devices.

For example, Huawei C8650 is an exception, running repack-bootimg.pl immediately after unpack-bootimg.pl result in a different boot.img — the scripts don’t take all possible arguments into consideration.

And after a little investigation, I found out a way to integrate a newly built kernel into your image, given that you already have Android built on your machine:

1. Use split_bootimg.pl to split an existing boot.img into boot.img-kernel and boot.img-ramdisk.gz.
2. Run in your shell:

   $ANDROID_SRC_DIR/out/host/linux-x86/bin/mkbootimg --kernel $NEW_KERNEL --ramdisk boot.img-ramdisk.gz --cmdline 'console=ttyDCC0 androidboot.hardware=huawei' --base 0x10200000 -o $NEW_IMG