add kexec support

[ddiss]

I'm working on xfstests CI jobs for SUSE and openSUSE kernels and plan on using the Open Build Service for kernel compilation. Given that obs compile jobs can be run within a VM, my plan is to boot the kernel directly after compilation via kexec.

[ddiss]

kexec support would also be useful for other CI systems where nested VMs aren't supported, e.g. Github hosted arm64 runners. Iit appears that kexec signature checks are used on those systems, so we'd need to deal with that.