Additional power control options

In configuration.h I have added two new defines.

#define PSU_DEFAULT_ON  //  Allows the user to set whether or not the
Marlin controlled PSU (ATX, Xbox, Other?) is turned on by default at
startup.  This define is enabled by default to retain functionality with
the main Marlin branch.  Comment out this define to leave the PSU off
when Marlin starts.

//#define LCD_POWER_CONTROL  //  This define enable menu entries to turn
your Marlin controlled power supply on and off.  Usefull if you want to
use your SD card and do some headless printing.  This define is disabled
by default to retain functionality with the main branch.