A corner case was identified where tasks with valid stamps from previous
builds need to be accounted for in the new sanity test in the migration
code. Add a variable to track such completed tasks to ensure the sanity
test works correctly.
(Bitbake rev: d517b1ef13ca7ab2fb4d761d3bd3b9fb7c591514)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>