sstate.bbclass: avoid deleting unrelated stamp files

sstate.bbclass: avoid deleting unrelated stamp files

Avoid deleting stamp files whose names contain the current task's name as a
substring. This will be especially important for example if do_package_write
is ever made an sstate task (as it would previously have deleted the stamps
here for do_package_write_ipk etc.)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>