10-stable does not boot on Dell E1505 any more

[Warren Block]

This is an older Dell E1505, Core 2 but does not have AHCI.

r255569 does not find /dev/ada0s2a and "?" at the mountroot> prompt 
shows no devices.

r255451 from September 10 boots fine.

Also possibly relevant is that after successfully building and 
installing world and kernel, /boot/kernel.old is not being replaced. 
This is good at present because the old kernel still works, but 
shouldn't kernel.old be replaced on installkernel?