mkdisk: Speed-up converting from raw image to qcow2

Author: mattiaswal

board/common/mkdisk.sh

@@ -213,5 +213,5 @@ genimage \
     --inputpath "$BINARIES_DIR" \
     --config "$root/genimage.cfg"
 
-qemu-img convert -c -O qcow2 "$tmpimage" "$BINARIES_DIR/$diskimg"
+qemu-img convert -c -m $(($(nproc) * 2)) -W -O qcow2 "$tmpimage" "$BINARIES_DIR/$diskimg"
 rm "$tmpimage"