Many times the value of the "install_devices" debconf setting in the grub-pc package is wrong for various reasons. Sometimes it's because Ubuntu's installer selected a different install device than the BIOS boots from, sometimes it's because the user manually selected that grub's boot sector should be installed to a partition (which is wrong, it should always go in the MBR), etc.

What is the officially supported way to change this debconf setting, preferably showing the user the helpful selection screen that used be given by sudo dpkg-reconfigure grub-pc (but which is no longer shown by this command in Ubuntu 12.04)?