env: Rename SYS_MMC_ENV_PART to ENV_MMC_EMMC_HW_PARTITION

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_EMMC_HW_PARTITION to clarify this is the eMMC hardware
partition selector, not a software partition table entry selector.
Retain the ENV_MMC_ prefix to make it easier to search for all the
SD/MMC related ENV options. 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 90fe9e7..5052149 100644
--- a/arch/arm/mach-imx/mx6/soc.c
+++ b/arch/arm/mach-imx/mx6/soc.c
@@ -540,10 +540,10 @@
 	return board_mmc_get_env_dev(devno);
 }
 
-#ifdef CONFIG_SYS_MMC_ENV_PART
+#ifdef CONFIG_ENV_MMC_EMMC_HW_PARTITION
 __weak int board_mmc_get_env_part(int devno)
 {
-	return CONFIG_SYS_MMC_ENV_PART;
+	return CONFIG_ENV_MMC_EMMC_HW_PARTITION;
 }
 
 uint mmc_get_env_part(struct mmc *mmc)
@@ -552,7 +552,7 @@
 
 	/* If not boot from sd/mmc, use default value */
 	if (devno < 0)
-		return CONFIG_SYS_MMC_ENV_PART;
+		return CONFIG_ENV_MMC_EMMC_HW_PARTITION;
 
 	return board_mmc_get_env_part(devno);
 }