ak2-core.sh: fix repacking of Sony ELF kernels

elftool fails to repack the ELF boot image on Sony devices when a custom dtb file is not provided within the AnyKernel2 zip, because the "dtb" variable, that contains the name of the dtb file, is overridden before the dtb filename is actually passed to the "rpm" variable. Swapping these two lines of code fixes the issue (tested by me on a Sony Xperia SP).

tools/ak2-core.sh

@@ -214,8 +214,8 @@ flash_boot() {
   done;
   if [ ! "$dtb" -a -f *-dtb ]; then
     dtb=`ls *-dtb`;
-    dtb="--dt $split_img/$dtb";
     rpm="$split_img/$dtb,rpm";
+    dtb="--dt $split_img/$dtb";
   fi;
   cd /tmp/anykernel;
   if [ -f "$bin/mkmtkhdr" ]; then