packagegroup: Make allarch inherit conditional

packagegroup: Make allarch inherit conditional

Make the allarch inherit conditional on PACKAGE_ARCH being left as
all. The one downside to this approach is that recipes need to set
PACKAGE_ARCH *before* inheriting the class in order to avoid the inherit.

The advantage is that we could start to detect the use of the
allarch inherit in the sstatesig code for improved task checksums.

(From OE-Core rev: 9c826962ec8fa45c2b035427442b90a41517144e)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>