Pi2 running 12.1 stops at loader prompt unbidden

[bob prohaska]

For some reason a Pi2 running 
12.1-STABLE r360240 GENERIC arm
stops at the loader prompt on the serial console:

[normal boot output]
.....

/boot/entropy size=0x1000

Hit [Enter] to boot immediately, or any other key for command prompt.


Type '?' for a list of commands, 'help' for more detailed help.

without any input on the serial console. No mouse nor keyboard.
Typing "boot" at the prompt continues successfully.

The machine just completed a complete buildworld/buildkernel/install
cycle so it should be reasonably up-to-date.

Have I got a configuration error someplace? By all prior 
experience it should have kept going.

Thanks for reading,

bob prohaska