Fix i386-pc prefix handling with nested partitions (Debian bug

Fix i386-pc prefix handling with nested partitions (Debian bug
#585068).  Note that the case where the core image is booted using
multiboot and relocated from its original location still requires
more work.
* kern/i386/pc/init.c (make_install_device): If the prefix starts
with "(,", fill the boot drive in between those two characters, but
expect that a full partition specification including partition map
names will follow.
* util/i386/pc/grub-setup.c (setup): Unless an explicit prefix was
specified, write a prefix without the drive name but including a
full partition specification.