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/env/mmc.c b/env/mmc.c
index 5451f91..dba99bd 100644
--- a/env/mmc.c
+++ b/env/mmc.c
@@ -164,7 +164,7 @@
 #if defined(CONFIG_ENV_MMC_PARTITION)
 	str = CONFIG_ENV_MMC_PARTITION;
 #else
-	/* look for the partition in mmc CONFIG_SYS_MMC_ENV_DEV */
+	/* look for the partition in mmc CONFIG_ENV_MMC_DEVICE_INDEX */
 	str = ofnode_conf_read_str(dt_prop.partition);
 #endif