cmd: env: add env select command

Add the new command 'env select' to force the persistent storage
of environment, saved in gd->env_load_prio.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index 498fd31..d7136b0 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -610,6 +610,11 @@
 	  Load all environment variables from the compiled-in persistent
 	  storage.
 
+config CMD_NVEDIT_SELECT
+	bool "env select"
+	help
+	  Select the compiled-in persistent storage of environment variables.
+
 endmenu
 
 menu "Memory commands"