sd-boot: when rebooting or powering off, save config state

sd-boot: when rebooting or powering off, save config state

The menu_run() function allows the user to set/unset default entry, or to
increase/decrease menu timeout. After a keypress, status like
  "Menu timeout set to 5 s"
is printed, but there actually isn't any immediate effect. The value is only
written right right before booting a menu entry to avoid unnecessary wear&tear
on the nvram storage. This delayed write is supposed to be invisible to the
user.

Nevertheless, operations like reboot into firmware, reboot, or shutdown were
done immediately. We need to exit the loop first, save the state, and only do
the op afterwards.

Fixup for f6531b11d21931b3952d566ceded672ba21681cc and
e6cab77eca8f6556f381c348b0452b526a752ab7.
Also reverts 498d0cc426afc13fdadb0a385fd16c005645e0cf.