Support bootmenu

Would it be possible to build hardkernel/u-boot with support for the `bootmenu` command (available since v2022.04):

https://u-boot.readthedocs.io/en/latest/usage/cmd/bootmenu.html#configuration

This would be very handy for kernel updates: If the new kernel does not boot for some reason, you could just reboot and select the old one again.