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.

(From OE-Core rev: 2c9b1d304daade7b0907320aeb9c522e7ab9dcab)

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>