image-container.bbclass: Error if not using linux-dummy
In the typical case of a container, the user doesn't particularly care
about the kernel version used. Since using linux-dummy prevents building
a kernel in the most common case, alert the user if they choose a
container image, but don't also specify linux-dummy.
In the cases the user actually does want to build against particular
kernel headers, etc, they can override by setting
IMAGE_CONTAINER_NO_DUMMY = "1".
[YOCTO #9502]
Signed-off-by: Randy Witt <randy.e.witt@linux.intel.com> Signed-off-by: Ross Burton <ross.burton@intel.com>
