reform-boot-config: allow messing up the currently running system

Add an interactive prompt that asks the user if they are sure that they want to rewrite the /boot partition of the currently running system to boot something else. While this will make the current system unbootable (and that's why the safeguard was introduced) making the current system unbootable is desired if the user boots of an SD-Card flashed with the sysimage and just wants to set up NVMe or eMMC and then use the SD-Card for /boot. It is thus undesirable to forbid reform-boot-config from making the current system unbootable as the user might be in the process of setting up a new system.