linux-dummy: add empty dependent packages

some kernel includes kernel-image-image and kernel-devicetree packages.
these patckages are defined in kernel.bbclass
but, when use linux-dummy, these packages are not defined.
so, define them as empty packages for avoiding compile error:

ERROR: Nothing RPROVIDES 'kernel-devicetree'

Signed-off-by: Li Wang <li.wang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>