classes: Be consistent about sstate-inputdirs/outputdirs ending with '/'

classes: Be consistent about sstate-inputdirs/outputdirs ending with '/'

If sstate-inputdirs and sstate-outputdirs don't match with ending '/'
characters, the manifest file can end up corrupted. This change
ensures the metadata is consistent in ending do_populate_root tasks
with this character to avoid manifest file corruption.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>