freescale: imx8mm_evk: Use IS_ENABLED instead of #ifdef

Use IS_ENABLED(CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG) to make the code
more readable and fix checkpatch.pl warning

Signed-off-by: Tommaso Merciai <tomm.merciai@gmail.com>
diff --git a/board/freescale/imx8mm_evk/imx8mm_evk.c b/board/freescale/imx8mm_evk/imx8mm_evk.c
index ab55135..e0975fc 100644
--- a/board/freescale/imx8mm_evk/imx8mm_evk.c
+++ b/board/freescale/imx8mm_evk/imx8mm_evk.c
@@ -60,9 +60,10 @@
 
 int board_late_init(void)
 {
-#ifdef CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG
-	env_set("board_name", "EVK");
-	env_set("board_rev", "iMX8MM");
-#endif
+	if (IS_ENABLED(CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG)) {
+		env_set("board_name", "EVK");
+		env_set("board_rev", "iMX8MM");
+	}
+
 	return 0;
 }