Support `kexec` when GRUB bootloader is not used

[MeteorsLiu]

In some environments, like @TencentCloud dones't use GRUB/GRUB2 bootloader, it won't create a bootloader option when we're trying to change /etc/default/grub or /boot/grub/grub.cfg.

However, it seems like an image bug. I have opened a ticket about this issue for that.

In that situation, we can use kexec instead, like

# load Debian installer
kexec -l /boot/debian-trixie/linux --initrd /boot/debian-trixie/initrd.gz

# start it right now, we can observe the progress via VNC
kexec -e