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/doc/device-tree-bindings/config.txt b/doc/device-tree-bindings/config.txt
index a617e7f..fffb69e 100644
--- a/doc/device-tree-bindings/config.txt
+++ b/doc/device-tree-bindings/config.txt
@@ -76,7 +76,7 @@
 	precedence. In that case, only if the partition is not found,
 	mmc-env-offset* will be tried.
 
-	Note that CONFIG_ENV_MMC_PARTITION overrides this device-tree setting.
+	Note that CONFIG_ENV_MMC_SW_PARTITION overrides this device-tree setting.
 
 u-boot,no-apm-finalize (bool)
 	For x86 devices running on coreboot, this tells U-Boot not to lock