- the user never answered the on which device(s) to install question
=> ask
- the user chose not to install grub
=> inform the user by the "You chose not to install GRUB" dialog and let him choose to install grub nonetheless now
- the user already chose where to install grub
=> do it
This should work on a new installation as well as on an upgrade from GRUB 1.
IMHO the debconf process should differentiate:
- the user never answered the on which device(s) to install question
=> ask
- the user chose not to install grub
=> inform the user by the "You chose not to install GRUB" dialog and let him choose to install grub nonetheless now
- the user already chose where to install grub
=> do it
This should work on a new installation as well as on an upgrade from GRUB 1.