mkimage alignment of initrd with -T multi

[Chris Packham]

Hi U-Boot,

I've been given a MIPS reference board with a fairly old vendor U-Boot
on it. It does seem to support the  Mutli-File Image format but I'm
running into an issue where the kernel expects the initrd on a page
boundary.

I'm using a command like the following to produce and image
  cat board.dtb >> vmlinux.bin
  gzip vmlinux.bin >vmlinux.gz
  mkimage -C gzip -A mips -O linux -T multi -a 0x80100000 -e
0x80100000 -d vmlinux.gz:initrd.sqsh myboard.img

I can mess about with padding vmlinux.gz after the gzip step but the
process is a bit fragile and involves me guessing at some magic
numbers that I think might be sizes of certain headers.

Is there any way to convince mkimage to make sure the initrd is on an
appropriately aligned boundary so I don't have to guess.

Thanks,
Chris