env: mmc: add debug message when mmc-env-partition is not found

Add a debug message to indicate a potential issue when
"u-boot,mmc-env-partition" is present in config node of device tree
but this partition name is not found in the mmc device.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
diff --git a/env/mmc.c b/env/mmc.c
index bd7d51e..8941e0f 100644
--- a/env/mmc.c
+++ b/env/mmc.c
@@ -120,6 +120,7 @@
 		err = mmc_offset_try_partition(str, copy, &val);
 		if (!err)
 			return val;
+		debug("env partition '%s' not found (%d)", str, err);
 	}
 
 	/* try the GPT partition with "U-Boot ENV" TYPE GUID */