rk3399: Check MMC env while defining it

rk3399 do support SPI flash as well, so there is
a possibility of using flash environment for those
usecases.

So define env device for MMC only when it is used
by specific configuration.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
diff --git a/include/configs/evb_rk3399.h b/include/configs/evb_rk3399.h
index b9c4d68..c0b0358 100644
--- a/include/configs/evb_rk3399.h
+++ b/include/configs/evb_rk3399.h
@@ -8,7 +8,9 @@
 
 #include <configs/rk3399_common.h>
 
-#define CONFIG_SYS_MMC_ENV_DEV 0
+#if defined(CONFIG_ENV_IS_IN_MMC)
+# define CONFIG_SYS_MMC_ENV_DEV		0
+#endif
 
 #define SDRAM_BANK_SIZE			(2UL << 30)