classes/buildhistory: write out task signatures on every build
If we want to determine what changed since the last build, one angle
from which to look at it is to check the signatures. However, if we
don't actually have the signatures from the last build we don't have
anywhere to start. Save the signatures on each build in order to give us
the starting point.
NOTE: you need to set your BUILDHISTORY_FEATURES value to include
"task" to enable collection of these signatures as it is is disabled by
default.
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
