Some cleanup around 'lcd_bootscreen()' and 'lcd_kill_screen()'

Move 'lcd_bootscreen()' from `lcd_init()` to 'setup()' where it is cexecute exactly once. Saves 'bool show_bootscreen'.
Move the call of 'lcd_custom_bootscreen()' to the begin of 'lcd_bootscreen()'.
Move the delays into the related functions.
Move the picture loop around 'lcd_kill_screen()' into the function.