The file 'kernel' gets copied to the generic_x86 image. Replace it with
the kernel from which you wish to boot. The same kernel is used for the
installer_dvi image as well as for the boot kernel.

To rebuild the kernel, use external/qemu/distrib/build-kernel.sh

/path/to/build-kernel.sh --arch=x86                -> full_x86 image (for QEMU)
/path/to/build-kernel.sh --arch=x86 --config=vbox  -> vbox_x86 image (for VirtualBox)

The kernel sources are in android.git.kernel.org/kernel/qemu.git,
in the android-goldfish-2.6.XX branch (where XX is the highest patch kernel version number).