mpc85xx: fix incorrect kernel target

mpc85xx: fix incorrect kernel target

The wrapper-image for the WL-WDR4900 was used as a build-target for the
kernel. This workd fine as long as only a single wrapper is used with
the OpenWrt build-system.

If additional wrappers are used, the build becomes racy in the
wrapper-stage.

The wrapper images actually do not represent a target. They are built
based on the kernel configuration. Only copy the resulting images to
avoid race-conditions as explained.

Signed-off-by: David Bauer <mail@david-bauer.net>