classes/externalsrc: fix source being wiped out on clean with kernel
kernel.bbclass adds ${S} do do_clean[cleandirs], but this means if you
run bitbake -c clean <kernelrecipe> then your external source tree will
be trashed, which could be a disaster. For safety, remove ${S} from
cleandirs for every task. We also have to do the same for ${B} in the
case where EXTERNALSRC_BUILD is set to the same value as EXTERNALSRC.
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
