Boards with no screen should not call BOARD_SCREEN = 1

[dexter4000]

I forked the code and modified it to integrate an ST7789 screen into the Feather M4 Express. Therefore, the board.mk is using

CHIP_FAMILY = samd51
CHIP_VARIANT = SAMD51J19A
SCREEN_TYPE = ST7789

But for boards without a screen, BOARD_SCREEN=1 should not be called. This way other screens can be added to the code later on.