classes/packagegroup: disable superfluous tasks

classes/packagegroup: disable superfluous tasks

Disable tasks other than do_package* (similar to how image.bbclass
disables everything except do_rootfs). This saves a little time as well
as neatly avoiding the LIC_FILES_CHKSUM verification within do_configure
which is not needed for package groups.

(From OE-Core rev: fde2b0db798f03cf297a0d466a9c3a444f1321f3)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>