env: mmc: statically set the environment partition name

The new opt-out setting, CONFIG_ENV_MMC_PARTITION, statically sets
the MMC environment partition name. Prior to this patch, the only way
to declare this partition name was by creating a
'u-boot,mmc-env-partition' parameter in the device-tree's /config node.

This setting provides additional flexibility, particularly in cases
where accessing the device-tree is not straightforward (e.g. QEMU).

If undeclared, the device-tree's setting will be used.

Signed-off-by: Emmanuel Di Fede <emmanuel.difede@cysec.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Patrick Delaunay <patrick.delaunay@foss.st.com>
diff --git a/doc/device-tree-bindings/config.txt b/doc/device-tree-bindings/config.txt
index 3151778..f50c68b 100644
--- a/doc/device-tree-bindings/config.txt
+++ b/doc/device-tree-bindings/config.txt
@@ -76,6 +76,8 @@
 	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.
+
 u-boot,no-apm-finalize (bool)
 	For x86 devices running on coreboot, this tells U-Boot not to lock
 	down the Intel Management Engine (ME) registers. This allows U-Boot to