env: unify logic to check and apply changes

The logic of checking special parameters (e.g. baudrate, stdin, stdout,
for a valid value and/or whether can be overwritten) and applying the
new value to the running system is now all within a single function
env_check_apply() which can be called whenever changes are made
to the environment, no matter if by set, default or import.

With this patch env_check_apply() is only called by "env set",
retaining previous behavior.

Signed-off-by: Gerlando Falauto <gerlando.falauto@keymile.com>
Reviewed-by: Marek Vasut <marex@denx.de>

include/search.h

@@ -99,6 +99,7 @@ extern int himport_r(struct hsearch_data *__htab,
 		     int __flag);
 
 /* Flags for himport_r() */
-#define	H_NOCLEAR	1	/* do not clear hash table before importing */
+#define	H_NOCLEAR	(1 << 0) /* do not clear hash table before importing */
+#define	H_FORCE		(1 << 1) /* overwrite read-only/write-once variables */
 
 #endif /* search.h */