[POWERPC] Make sure initrd and dtb sections get into zImage correctly

The "wrapper" script was using the wrong names for the initrd and
dtb (device-tree blob) sections.  This fixes it, and also ensures
the symbols for the start and end of the dtb get defined correctly.

Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/boot/zImage.lds.S

@@ -21,6 +21,11 @@ SECTIONS
     __got2_end = .;
   }
 
+  . = ALIGN(8);
+  _dtb_start = .;
+  .kernel:dtb : { *(.kernel:dtb) }
+  _dtb_end = .;
+
   . = ALIGN(4096);
   _vmlinux_start =  .;
   .kernel:vmlinux.strip : { *(.kernel:vmlinux.strip) }