env: Rename setenv() to env_set()

We are now using an env_ prefix for environment functions. Rename setenv()
for consistency. Also add function comments in common.h.

Suggested-by: Wolfgang Denk <wd@denx.de>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/net/eth_common.c b/net/eth_common.c
index 58fa295..ef97da7 100644
--- a/net/eth_common.c
+++ b/net/eth_common.c
@@ -39,7 +39,7 @@
 
 	sprintf(buf, "%pM", enetaddr);
 
-	return setenv(name, buf);
+	return env_set(name, buf);
 }
 
 int eth_getenv_enetaddr_by_index(const char *base_name, int index,
@@ -97,14 +97,14 @@
 	/* update current ethernet name */
 	if (eth_get_dev()) {
 		if (act == NULL || strcmp(act, eth_get_name()) != 0)
-			setenv("ethact", eth_get_name());
+			env_set("ethact", eth_get_name());
 	}
 	/*
 	 * remove the variable completely if there is no active
 	 * interface
 	 */
 	else if (act != NULL)
-		setenv("ethact", NULL);
+		env_set("ethact", NULL);
 }
 
 void eth_try_another(int first_restart)