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/main.c b/common/main.c
index 2116a9e..07fc31b 100644
--- a/common/main.c
+++ b/common/main.c
@@ -48,7 +48,7 @@
 	bootstage_mark_name(BOOTSTAGE_ID_MAIN_LOOP, "main_loop");
 
 #ifdef CONFIG_VERSION_VARIABLE
-	setenv("ver", version_string);  /* set version variable */
+	env_set("ver", version_string);  /* set version variable */
 #endif /* CONFIG_VERSION_VARIABLE */
 
 	cli_init();