externalsrc.bbclass: better filtering of cleandirs

externalsrc.bbclass: better filtering of cleandirs

We really do not want our (external) source tree to be removed. There
might be multiple values in the 'cleandirs' varflag pointing to our
source tree - causing it to be wiped out. This patch improves the
filtering of 'cleandirs' by examining the expanded values inside it. Any
(expanded) values pointing to our source tree get removed.

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>