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/include/mmc.h b/include/mmc.h
index 87f7ef1..c6b2ab4 100644
--- a/include/mmc.h
+++ b/include/mmc.h
@@ -996,7 +996,7 @@
 int board_mmc_init(struct bd_info *bis);
 int cpu_mmc_init(struct bd_info *bis);
 int mmc_get_env_addr(struct mmc *mmc, int copy, u32 *env_addr);
-# ifdef CONFIG_SYS_MMC_ENV_PART
+# ifdef CONFIG_ENV_MMC_EMMC_HW_PARTITION
 extern uint mmc_get_env_part(struct mmc *mmc);
 # endif
 int mmc_get_env_dev(void);