nativesdk-buildtools-perl-dummy.bb: Fix variable expansion in python code
Since python expansion of bb data variables is disable in order to
provide a standard interface usage for expand variables this variables
aren't expanded now so change to call d.expand('${VAR}').
This API expansion change was causing to install perl in nativesdk.
Signed-off-by: Aníbal Limón <anibal.limon@linux.intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
