kexec: DTB not found after executing new kernel

This one is pretty simple - kexec doesn't work properly on our current kernels, usually giving an error about the DTB being corrupt or invalid, but sometimes doing Weird:tm: things like filling the framebuffer with garbage.

It's worth noting that the new kernel uses linux-loader's IPC earlyboot print mechanism, which wasn't intended to run after the kernel was already going and overwrites memory in the framebuffer. This could be related, but I think it's a red herring as corruption is usually not visible, so we're not overwriting the DTB or anything.

Adding to Distro-ready milestone because a working kexec would allow petitboot to work, solving some of our bootloader issues.