Add dt.img into BOARD_PACK_RADIOIMAGES.

So it will be included into hikey960-img.zip.

Bug: 62195105
Test: lunch hikey960-userdebug; m dist. Check hikey960-img.zip.
Change-Id: I3309da84ea66b101daa2b84fcfeb4d71bf6b35d1
diff --git a/build/tasks/dtimage.mk b/build/tasks/dtimage.mk
index 9dc6668..4d6a74d 100644
--- a/build/tasks/dtimage.mk
+++ b/build/tasks/dtimage.mk
@@ -8,5 +8,10 @@
 	$(MKDTIMG) -c -d $(DTB) -o $@
 
 droidcore: $(PRODUCT_OUT)/dt.img
+
+# Images will be packed into target_files zip, and hikey-img.zip.
+INSTALLED_RADIOIMAGE_TARGET += $(PRODUCT_OUT)/dt.img
+BOARD_PACK_RADIOIMAGES += dt.img
+
 endif
 endif