archive-*-source.bbclass: have do_dumpdata_create_diff_gz task run before do_rootfs

archive-*-source.bbclass: have do_dumpdata_create_diff_gz task run before do_rootfs

do_rootfs[cleandirs] contains ${S} and, if do_rootfs task starts before
the do_dumpdata_create_diff_gz is finished, an error will occur in the
process because the directory will be removed while still needed by the
create_diff_gz() function.

This patch will force the do_dumpdata_create_diff_gz task to run before
do_rootfs when the final image is created.

[YOCTO #4310]

Signed-off-by: Laurentiu Palcu <laurentiu.palcu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>