Looking at /usr/share/kodi/addons/skin.estuary/xml/DialogButtonMenu.xml we can see that the button will only show up if there are a /dev/system or a /dev/userdata file.
<item>
<label>Reboot from eMMC/NAND</label>
<onclick>System.ExecWait("/usr/sbin/rebootfromnand")</onclick>
<onclick>Reset()</onclick>
<visible>System.PathExist("/dev/system") | System.PathExist("/dev/userdata")</visible>
</item>
Apparently (latest nightly-ng here), no such files are present for me, so the condition is just plain wrong for my device or build (?).
CoreELEC:/usr/share/kodi # ls /dev/system
ls: /dev/system: No such file or directory
CoreELEC:/usr/share/kodi # ls /dev/userdata
ls: /dev/userdata: No such file or directory
Until a proper fix is done, i copied the estuary skin to ~/.kodi/addons, removed the line, bumped the skin version to an arbitrary 5.0.0 in addon.xml and voilà, the button appears !
$ cp -R /usr/share/kodi/addons/skin.estuary/ ~/.kodi/addons/skin.estuary
$ # EDIT xml/DialogButtonMenu.xml
$ grep version addon.xml | grep name
<addon id="skin.estuary" version="5.0.0" name="Estuary" provider-name="phil65, Ichabod Fletchman">
Anyone knows how to do this properly ? (i.e. match the condition or fix the condition)