Fix tr and dd for macOS

See Unix Stack Exchange question "Filling file with 0xFF gives C3BF in
OSX" at https://superuser.com/a/1349497/150212

and

https://lists.freebsd.org/pipermail/freebsd-questions/2007-October/160961.html

bootloader/build_multi_image.sh

@@ -6,8 +6,8 @@ BIN1="$2"
 BIN2="$3"
 BINOUT="$4"
 
-dd if=/dev/zero ibs=1k count=448 2>/dev/null | tr "\000" "\377" > "$BINOUT"
+dd if=/dev/zero ibs=1k count=448 2>/dev/null | LANG=C tr "\000" "\377" > "$BINOUT"
 dd if="$BIN1" of="$BINOUT" conv=notrunc 2>/dev/null
-dd if="$BIN2" of="$BINOUT" conv=notrunc oflag=append 2>/dev/null
+dd if="$BIN2" >> "$BINOUT" 2>/dev/null
 
 "$PYTHON" "$(dirname "$0")/crc_patch.py" "$BINOUT"