classes/externalsrc: ensure cleandirs code handles non-absolute paths

classes/externalsrc: ensure cleandirs code handles non-absolute paths

It's possible that a trailing or extra slash somewhere in the external
source path could result in the directory not being removed from
cleandirs; it's also possible that a cleandirs entry is somewhere
underneath the source tree and that tree should never have parts of it
deleted by the build system. Use oe.path.is_path_parent() (which makes
paths absolute before checking them) to find out if any path in
cleandirs is anywhere underneath the external source path, and drop it
if it is.

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