env: Rename ENV_MMC_PARTITION to ENV_MMC_SW_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_SW_PARTITION to clarify this is the SD/MMC software
partition table entry selector. Update the help text accordingly.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
diff --git a/env/mmc.c b/env/mmc.c
index 5d1cb78..11375bd 100644
--- a/env/mmc.c
+++ b/env/mmc.c
@@ -161,8 +161,8 @@
 	hwpart = mmc_get_env_part(mmc);
 #endif
 
-#if defined(CONFIG_ENV_MMC_PARTITION)
-	str = CONFIG_ENV_MMC_PARTITION;
+#if defined(CONFIG_ENV_MMC_SW_PARTITION)
+	str = CONFIG_ENV_MMC_SW_PARTITION;
 #else
 	/* look for the partition in mmc CONFIG_ENV_MMC_DEVICE_INDEX */
 	str = ofnode_conf_read_str(dt_prop.partition);