common: add command power off - to switch off the device by command

common: add command power off - to switch off the device by command

This change introduces new config:
- CONFIG_CMD_POWEROFF - which enables common/cmd_poweroff.c

This requires implementation of function board_poweroff() which
is yet declared in include/common.h

Implementation of board_poweroff() should:
1.a. turn off the device
     or
1.b. print info to user about turn off ability
2.   never back to caller

Usage is simple: "power off"

Change-Id: Ia5fe73250e2ac29d0868b80bcd867bae2aa8d5be
Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>