env: Rename SYS_MMC_ENV_PART to ENV_MMC_EMMC_HW_PARTITION

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>