u8g2: use libtock exists() check instead of syscall

Author: ppannuto

u8g2/u8g2-tock.c

@@ -75,7 +75,7 @@ static uint8_t u8x8_d_ssd1306_tock(u8x8_t *u8x8,
 // Initialize the u8g2 library for Tock use. Call this before using the rest of
 // the library.
 int u8g2_tock_init(u8g2_t *u8g2) {
-  if (!driver_exists(DRIVER_NUM_SCREEN)) {
+  if (!libtocksync_screen_exists()) {
     return -1;
   }