packagegroup.bbclass: Use immediate expanded PACKAGE_ARCH value

packagegroup.bbclass: Use immediate expanded PACKAGE_ARCH value

When the PACKAGE_ARCH is dependant of a override, the expanding is
done too late triggering the false-positive error of wrong inclusion
order.

To fix this we use immediate expansion operator to evaluate the
PACKAGE_ARCH value in order to choose to load 'allarch' class or not.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>