grub: support 'guest disks'

[Yotam Medini]

Hello Grub gurus,

I may have missed some Grub feature or Grub hidden documentation note.
If so, please let me know.

Say, you have two computers C1 and C2, with hard disks H1 in C1
and H2 in C2. Now C1 has linux nicely installed on its H1, 
while C2 & H2 are almost empty.

Say you want to install grub boot on H2 for C2.  It will be convenient
to unscrew some screws and bring H2 as a (secondary) disk to C1, and
use C1 to install grub in H2.  After successful installation (lots of
chroot operations) and configuration, bring back H2 to C3.

Now, when writing the grub menu for H2, while at C1, 
you end up referring to (hd1,.),
while you actually want to finally have (hd0,.) when back at H2.
A nice feature would be, to have an implicit symbol, 
for the "device-being-booted-by", say 'bd'.

Then in a grub menu, you could specify - for example:

   title=Boot Linux root (bd,4) 
   kernel /boot/bzImage root=/dev/bd5

Which will be equivalent to:

 * When secondary on C1

   title=Boot Linux root (hd1,4) 
   kernel /boot/bzImage root=/dev/hdb5

 * When primary on C2

   title=Boot Linux root (hd0,4) 
   kernel /boot/bzImage root=/dev/hda5


-- yotam