avoid auto power-off in u-boot mode

[Javi Roman]

The entry http://wiki.openmoko.org/wiki/Flashing_openmoko#Preparing_the_device_for_DFU_mode
warns us about some devices turns off after a period of inactivity
while on the u-boot menu.

I am not really sure if this issue is hardware related, because the
same behavior happens when we use QEMU to go into u-boot: If you log
into u-boot by means of "telnet localhost 1200", the system power-off
passed a few seconds, therefore is difficult to work under these
conditions.

Is it possible to avoid this nuisance?

Regards,

        -Javi