Create grub-<platform>-bin packages corresponding to all grub-<platform>

Create grub-<platform>-bin packages corresponding to all grub-<platform>
packages (except for grub-emu).  These do not automatically install the
boot loader or update grub.cfg, and they install their binaries to
/usr/lib/grub/<cpu>-<platform>/; this means that can be installed in
parallel, making it easier to use them to build GRUB-based disk images
(e.g. d-i).  The grub-<platform> packages now depend on these and
include symlinks, so their behaviour will remain as before.