powerpc/bootwrapper: add missing bit of simpleImage target

The wrapper script is missing the bits needed for building generic
simpleImage targets (targets which don't depend on any particular
firmware interface and retrieve all their data from the device tree).
Signed-off-by: NGrant Likely <grant.likely@secretlab.ca>

arch/powerpc/boot/wrapper

@@ -214,6 +214,10 @@ simpleboot-virtex440-*)
     platformo="$object/simpleboot.o $object/virtex.o"
     binary=y
     ;;
+simpleboot-*)
+    platformo="$object/simpleboot.o"
+    binary=y
+    ;;
 asp834x-redboot)
     platformo="$object/fixed-head.o $object/redboot-83xx.o"
     binary=y