boltd does not handle empty sysfs key attribute
If one enables pre-boot authorization and also secure mode, then the sysfs "key
" file may be empty. This causes boltctl to fail to enroll a device in this state, i.e.
# boltctl enroll 00a6f9XX-XXXX-XXXX-ffff-ffffffffffff
could not determine existing authorizationunexpected key size (corrupt
key?): 1
Ideally, the device should be enrolled with a new key, but that probably just cannot work. So really, we probably just can throw a better error message, effectively saying that enroll is only possible if the device has not been authorized yet.