Trigger emergency shutdown on resume when the boot device is missing

Originally created by @intrigeri on #16787 (Redmine)

i.e. it was removed during suspend. The idea is that when suspended, both sequences of user actions should result in emergency shutdown:

wake up the system, then unplug the boot device: this works
unplug the boot device, then wake up the system: this used to work but the fix for #11729 (closed) broke it as a side effect (e4e7f812)

Edited May 21, 2020 by intrigeri

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