Enhanced SD card handling:
1) If LCD support enabled but LCD not connected allow use of controller SD slot.
2) If LCD support enabled and LCD connected allow use of either SD slot.
In the case that both SD slots have a card inserted, the LCD SD slot has precedence.
Prevent SD card insertion from causing a hang when using LiquidTWI2 device detection
Merge branch 'lcd_detected' of https://github.com/whosawhatsis/Marlin into Marlin_v1_Mendel90