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>

include/env.h

@@ -286,6 +286,13 @@ int env_save(void);
  */
 int env_erase(void);
 
+/**
+ * env_select() - Select the environment storage
+ *
+ * @return 0 if OK, -ve on error
+ */
+int env_select(const char *name);
+
 /**
  * env_import() - Import from a binary representation into hash table
  *
@@ -349,5 +356,4 @@ int env_get_char(int index);
  * This is used for those unfortunate archs with crappy toolchains
  */
 void env_reloc(void);
-
 #endif