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/imx8ulp/soc.c b/arch/arm/mach-imx/imx8ulp/soc.c
index b5dc452..ab1e1bc 100644
--- a/arch/arm/mach-imx/imx8ulp/soc.c
+++ b/arch/arm/mach-imx/imx8ulp/soc.c
@@ -56,7 +56,7 @@
 
 	if (ret != ROM_API_OKAY) {
 		puts("ROMAPI: failure at query_boot_info\n");
-		return CONFIG_SYS_MMC_ENV_DEV;
+		return CONFIG_ENV_MMC_DEVICE_INDEX;
 	}
 
 	boot_type = boot >> 16;
@@ -64,7 +64,7 @@
 
 	/* If not boot from sd/mmc, use default value */
 	if (boot_type != BOOT_TYPE_SD && boot_type != BOOT_TYPE_MMC)
-		return env_get_ulong("mmcdev", 10, CONFIG_SYS_MMC_ENV_DEV);
+		return env_get_ulong("mmcdev", 10, CONFIG_ENV_MMC_DEVICE_INDEX);
 
 	return board_mmc_get_env_dev(boot_instance);
 }