classes/externalsrc: handle if cleandirs contains python expressions

classes/externalsrc: handle if cleandirs contains python expressions

Use the existing oe.recipeutils.split_var_value() function to split the
unexpanded value of the cleandirs varflag, in case it contains python
expressions - we don't want to split the expression itself as the chunks
will not expand properly individually and we can miss something that
expands to the source tree (and thus it can get deleted, the avoidance
of which is the whole point of this code).

(From OE-Core rev: 611cee69d7300a4746edd752c9557af60e7beecc)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>