env: Rename SYS_MMC_ENV_DEV to ENV_MMC_DEVICE_INDEX

Rename the variable and add ENV_ prefix, so that all configuration
options which are related to environment would have an CONFIG_ENV_
prefix. No functional change.

Use ENV_MMC_DEVICE_INDEX to clarify this is the SD/MMC device
index, a number, as enumerated by U-Boot. Update the help text
accordingly.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
diff --git a/arch/arm/mach-imx/mx6/soc.c b/arch/arm/mach-imx/mx6/soc.c
index d4a6173..90fe9e7 100644
--- a/arch/arm/mach-imx/mx6/soc.c
+++ b/arch/arm/mach-imx/mx6/soc.c
@@ -502,7 +502,7 @@
 #ifdef CONFIG_ENV_IS_IN_MMC
 __weak int board_mmc_get_env_dev(int devno)
 {
-	return CONFIG_SYS_MMC_ENV_DEV;
+	return CONFIG_ENV_MMC_DEVICE_INDEX;
 }
 
 static int mmc_get_boot_dev(void)
@@ -535,7 +535,7 @@
 
 	/* If not boot from sd/mmc, use default value */
 	if (devno < 0)
-		return CONFIG_SYS_MMC_ENV_DEV;
+		return CONFIG_ENV_MMC_DEVICE_INDEX;
 
 	return board_mmc_get_env_dev(devno);
 }