Marlin v1 - BUG FIXES / CORRECTIONS

Melzi support

Fixes #153 for bad call to abs()

This reverts commit 3682d9bd5b.

This is required when moving the buttons_init() function to the
lcd_init() function

Since the class "MainMenu" was used within a static variable the
initialization of the object (constructor call) was done before Arduino
library startup. It always caused a crash when using AVRStudio with
JTAG debugger (caused from calling the LCD initialization / the lot of
I/O work / the stack used during this calls). By moving the LCD_INIT
out of the constructor and using an explicit call inside of Arduino
setup() implementation immediately fixed all problems and the JTAG
debugger runs fine.

"i" runs from 0 to 4 but "add_homeing" array size is 3 only. On the
other hand the calculated value gets overwritten by either one of the
if choice.

Added 2nd extruder temperature LCD readout and setting.

adding M120 and 121 for turning on and off the endstops only for homing

Fix to allow M120/M121

Fix compile error on boards without a bed heater.

I could not find why this problem was introduced. 
Its quite obvious, so I think its in general interest to merge.

Since a removal or insert of the sd card takes a long time in firmware, just reinitializing the lcd does not hurt.
actually, it solves a problem with the UltiControlle

see https://github.com/ErikZalm/Marlin/issues/148

make should rebuild if configuration or Makefile is changed

Changed SLOWDOWN. IF this does not work ok OLD_SLOWDOWN is the old algo.