Some users may build for different archs under same workdir, so they
don't want to clean the dirs not related to current arch.
Run command 'bitbake -e' with selected packages to get the dirs related
to current arch then clean them.
Update the way to get the WORKDIR by parsing the IMAGE_ROOTFS by the
way.
(From OE-Core rev: a16727ebc341e0a0ce59a5200dc774cf672593ee)
Signed-off-by: Kang Kai <kai.kang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>