gui: Added glue logic for LittlevGL GUI library

Added glue logic to interface Zephyr with LittlevGL GUI library

This includes:
 * KConfig options for all lvgl options
 * Kernel & user space memory management
 * Zephyr to lvgl FS call mapping
 * Color space conversion function

Signed-off-by: Jan Van Winkel <jan.van_winkel@dxplore.eu>