cmd: spl: fix compiling error when CONFIG_CMD_SPL_WRITE_SIZE not defined

CONFIG_CMD_SPL_WRITE_SIZE is used for writing parameters to non-volatile
storage. So far it is only used for NAND. Fix compiling error when this
macro is not used for SD.

Signed-off-by: York Sun <york.sun@nxp.com>
CC: Anatolij Gustschin <agust@denx.de>

cmd/spl.c

@@ -121,9 +121,11 @@ static int spl_export(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
 				(void *)images.ft_addr);
 			env_set_addr("fdtargsaddr", images.ft_addr);
 			env_set_hex("fdtargslen", fdt_totalsize(images.ft_addr));
+#ifdef CONFIG_CMD_SPL_WRITE_SIZE
 			if (fdt_totalsize(images.ft_addr) >
 			    CONFIG_CMD_SPL_WRITE_SIZE)
 				puts("WARN: FDT size > CMD_SPL_WRITE_SIZE\n");
+#endif
 			break;
 #endif
 		case SPL_EXPORT_ATAGS: