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/common/update.c b/common/update.c
index 0767fcb..061a554 100644
--- a/common/update.c
+++ b/common/update.c
@@ -67,7 +67,7 @@
 	tftp_timeout_count_max = cnt_max;
 
 	/* we don't want to retry the connection if errors occur */
-	setenv("netretry", "no");
+	env_set("netretry", "no");
 
 	/* download the update file */
 	load_addr = addr;
@@ -83,7 +83,7 @@
 	tftp_timeout_ms = saved_timeout_msecs;
 	tftp_timeout_count_max = saved_timeout_count;
 
-	setenv("netretry", saved_netretry);
+	env_set("netretry", saved_netretry);
 	if (saved_netretry != NULL)
 		free(saved_netretry);