Upstream do not have a documented or scripted example that "Just Works" that includes a kernel and rootfs image build.

From experience to this point, the best approach appears to be:

1. Break out ignite/images/kernel/ into a project weaveworks/firecracker-kernels
2. gitsubrepoclone weaveworks/firecracker-kernels.git images/kernel in your current sources.
3. PR to Firecracker-Demo to add the weaveworks/firecracker-kernels project to the README.md
4. PR to Firecracker to replace RootFS and Kernel Setup / Creating a Kernel Image with a link to
   weaveworks/firecracker-kernels. This would close upstream issues #1724, and arguably properly address/close issues #824

This would have the (IMO) benefit of giving your work the prominence it deserves - I don't see anyone else openly trying to build different kernels for firecracker.