spl: ti: Avoid serial calls when serial support is disabled

If CONFIG_SPL_SERIAL_SUPPORT is not set, then the build will fail:

board/ti/am335x/built-in.o: In function `spl_start_uboot':
board/ti/am335x/board.c:247: undefined reference to `serial_tstc'
board/ti/am335x/board.c:247: undefined reference to `serial_getc'

Avoid the calls to the serial functions in that case.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>

board/ti/am335x/board.c

@@ -243,9 +243,11 @@ static struct emif_regs ddr3_icev2_emif_reg_data = {
 #ifdef CONFIG_SPL_OS_BOOT
 int spl_start_uboot(void)
 {
+#ifdef CONFIG_SPL_SERIAL_SUPPORT
 	/* break into full u-boot on 'c' */
 	if (serial_tstc() && serial_getc() == 'c')
 		return 1;
+#endif
 
 #ifdef CONFIG_SPL_ENV_SUPPORT
 	env_init();