Plat DA15000: Update the output file name in the "imgprep" script. (#1661)

diff --git a/third_party/dialog/DialogTools/imgprep.sh b/third_party/dialog/DialogTools/imgprep.sh
index 8dce2ee..da20603 100755
--- a/third_party/dialog/DialogTools/imgprep.sh
+++ b/third_party/dialog/DialogTools/imgprep.sh
@@ -1,4 +1,4 @@
-arm-none-eabi-objcopy -O binary ../../../output/da15000/bin/arm-none-eabi-ot-cli-ftd ../../../output/da15000/bin/arm-none-eabi-ot-cli-ftd.bin
+arm-none-eabi-objcopy -O binary ../../../output/da15000/bin/ot-cli-ftd ../../../output/da15000/bin/ot-cli-ftd.bin