[i.MX8] make spl_board_boot_device overwritable by specific board implementation

board/seco/common/board_imx8.c

@@ -83,7 +83,8 @@ void boot_validate (int gpio, const iomux_v3_cfg_t gpio_pad) {
  * |                                 BOOT DEVICE                              |
  * |__________________________________________________________________________|
  */
-int spl_board_boot_device(enum boot_device boot_dev_spl) {
+//default method that can be overwritten
+__weak int spl_board_boot_device(enum boot_device boot_dev_spl) {
 	switch (boot_dev_spl) {
 		case MMC1_BOOT:
 			return BOOT_DEVICE_MMC1;