The DISK_SIGNATURE variable is documented but has been removed since it was introduced.
I removed this from the documentation. The only area was the variables glossary in the ref-manual. I am setting to RESOLVED and putting the doc flag to "done" as the fix should not need reviewing. If you want to look, see the glossary and the "DESCRIPTION" variable (https://yoctoproject.org/docs/2.7/ref-manual/ref-manual.html#var-DESCRIPTION), which was just in front of where "DISK_DESCRIPTION" used to be. Note that "DISK_DESCRIPTION" no longer exists. Thanks, Scott