Issue 291: Add "suspend" command

Reported by Nikolaus Schaller, Nov 7, 2011

Add a command similar to "halt" but that does not power 
down through the TPS but sends the processor in Sleep mode after 
enabling the AUX button and maybe some sensors (BMA180) for wakeup.

This can be used to measure sleep mode power demand and test the 
wakeup features.

Comment 1 by Nikolaus Schaller, Dec 20, 2011

Will be named "suspend" since "sleep" is already 
defined.
The command is already defined but there is no code that enables 
devices for wakeup and sends the CPU to suspend.

Summary: Add "suspend" command

LetuxOS U-Boot