genimage.bbclass: Do not generate SSTATE artifacts

The artifacts created would be quite large and keeping multiple of them
will consume a notable amount of disk space while the expected speed-up
is really minimal.
(For the same reasons, the image class does not generate sstate
artifacts).

Having SSTATE artifacts enabled here before also led to
unexpected/unwanted behavior where a rootfs image recipe was rebuild
and the binary output (file system image) changed but did not make it
into the final genimage-generated disk image.
This becomes a major problem, e.g. in the context of verified boot.

Signed-off-by: Enrico Jörns <ejo@pengutronix.de>

classes-recipe/genimage.bbclass

@@ -186,6 +186,7 @@ fakeroot do_genimage () {
 }
 do_genimage[depends] += "virtual/fakeroot-native:do_populate_sysroot"
 do_genimage[prefuncs] += "do_genimage_preprocess"
+SSTATE_SKIP_CREATION:task-genimage = '1'
 
 addtask genimage after do_configure