fix(build_multi_image): Use objcopy instead of tr hack

The tr hack we used so far has been painful.  The amount of locale
environment variables we need to set is surely a telling sign of that ...

Replace this with objcopy which can do the same thing, but much easier
and without locale related side effects ;)

Signed-off-by: Rahix <rahix@rahix.de>

bootloader/build_multi_image.sh

@@ -6,8 +6,7 @@ BIN1="$2"
 BIN2="$3"
 BINOUT="$4"
 
-dd if=/dev/zero ibs=1k count=448 2>/dev/null | LANG=C LC_CTYPE=C LC_ALL=C LC_COLLATE=C tr "\000" "\377" > "$BINOUT"
-dd if="$BIN1" of="$BINOUT" conv=notrunc 2>/dev/null
-dd if="$BIN2" >> "$BINOUT" 2>/dev/null
+objcopy -I binary -O binary --pad-to=458752 --gap-fill=255 "${BIN1}" "$BINOUT"
+cat "$BIN2" >>"$BINOUT"
 
 "$PYTHON" "$(dirname "$0")/crc_patch.py" "$BINOUT"