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/doc/device-tree-bindings/config.txt b/doc/device-tree-bindings/config.txt
index f50c68b..a617e7f 100644
--- a/doc/device-tree-bindings/config.txt
+++ b/doc/device-tree-bindings/config.txt
@@ -70,7 +70,7 @@
 u-boot,mmc-env-partition (int)
 	if present, the environment shall be placed at the last
 	CONFIG_ENV_SIZE blocks of the partition on the
-	CONFIG_SYS_MMC_ENV_DEV.
+	CONFIG_ENV_MMC_DEVICE_INDEX.
 
 	if u-boot,mmc-env-offset* is present, this setting will take
 	precedence. In that case, only if the partition is not found,