Support `systemctl kexec` reboots

I know you are playing with the idea of a `systemd-boot` implementation of the boot chain instead of grub, and as such I am filing this feature request to help inform you on design!

**What I'd like:** We are running on bare metal, and as such there is a very lengthy firmware boot up time (several minutes). It would be beneficial if Bottlerocket supported a `kexec` reboot into the new root.
This would be a bit of a niche use-case, as `systemctl kexec` is not widely used, and it should be known that `kexec` reboots currently don't understand UKIs (which generally would be a great choice), but there has been a patch submitted [GH](https://github.com/systemd/systemd/issues/28538), [kernel.org mailing list](https://lore.kernel.org/kexec/20230911052535.335770-1-kernel@jfarr.cc/T/)

**Any alternatives you've considered:** Suffer through the painfully long UEFI boot times.