Only attempt to load /etc/random.seed from the boot device after the kernel

Only attempt to load /etc/random.seed from the boot device after the kernel
image has been succesfully loaded (with the recent loadfile changes allowing
us to know where the randomness needs to be loaded). While there, don't
bother doing this when booting from tape.

This works around the sun4e PROM 1.6, which gets confused by PROM open() -
close() sequences without any I/O happening in between.

Crank boot blocks version to 2.11.