at_bblock
apt.at_bblock = NO_BOOT_BLOCK;
apt.at_bblock = blkno; /* got it */
apt.at_bblock == NO_BOOT_BLOCK)
apt.at_bblock = blkno;
have_root = (apt->at_bblock != NO_BOOT_BLOCK);
if (pd->ap_st == apt->at_bblock)