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/board/st/stm32mp2/stm32mp2.c b/board/st/stm32mp2/stm32mp2.c
index 2547f2e..a72056e 100644
--- a/board/st/stm32mp2/stm32mp2.c
+++ b/board/st/stm32mp2/stm32mp2.c
@@ -183,7 +183,7 @@
 
 int mmc_get_env_dev(void)
 {
-	const int mmc_env_dev = CONFIG_IS_ENABLED(ENV_IS_IN_MMC, (CONFIG_SYS_MMC_ENV_DEV), (-1));
+	const int mmc_env_dev = CONFIG_IS_ENABLED(ENV_IS_IN_MMC, (CONFIG_ENV_MMC_DEVICE_INDEX), (-1));
 
 	if (mmc_env_dev >= 0)
 		return mmc_env_dev;