As rm_work is just cleanup it shouldn't starve more important tasks such as
do_compile of I/O, so use BB_TASK_IONICE_LEVEL to run the task in the idle
scheduler class.
(From OE-Core rev: 6025a14dbbd09b2805fe2e17ddc24f2a515cb832)
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>