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/env/mmc.c b/env/mmc.c
index 7a5836a..cb14bbb 100644
--- a/env/mmc.c
+++ b/env/mmc.c
@@ -114,8 +114,13 @@
 	if (IS_ENABLED(CONFIG_SYS_MMC_ENV_PART))
 		hwpart = mmc_get_env_part(mmc);
 
+#if defined(CONFIG_ENV_MMC_PARTITION)
+	str = CONFIG_ENV_MMC_PARTITION;
+#else
 	/* look for the partition in mmc CONFIG_SYS_MMC_ENV_DEV */
 	str = ofnode_conf_read_str(dt_prop.partition);
+#endif
+
 	if (str) {
 		/* try to place the environment at end of the partition */
 		err = mmc_offset_try_partition(str, copy, &val);