ui/ncurses: Don't modify config when clearing IPMI override

When safe mode is active the config displayed in nc-config is only a
subset of the actual config since device init has not yet occurred.
However when the "clear override" checkbox is ticked and the config
saved, the form will set the config as it is displayed, resulting in
device-specific config (eg. boot order and network settings) being
cleared. If the user only ticked the "clear override" checkbox this most
likely isn't what they intended.

Instead change the checkbox to a button which when pressed clears the
override and exits safe mode if active, without modifying the rest of
the configuration.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>
(cherry picked from commit c30d76aabf5f548556ea25b1fac5eb707a9f737a)