We don't need to process the holdoff task list until we're executing tasks
which saves some data manipulation, at the cost of some data structures
not being correct at all times. This saves significant amounts of time
in various profile charts of larger builds.
(Bitbake rev: 270f076111b12eab358417b0c4cf9c70d7cc787a)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>