In single testing with a shutdown scenario the processes are cleaned up correctly
but the manual cleanup fall back used for a minimal image do not work properly.
This patch fixes the kill commands to revert to non-process groups, fixing
the hung process issues that were occuring.
(From OE-Core rev: 1c7d4fc88a9e7a3cdb41044ce7eedadd38e99952)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>