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/board/udoo/neo/neo.c b/board/udoo/neo/neo.c
index 276c625..7fa1289 100644
--- a/board/udoo/neo/neo.c
+++ b/board/udoo/neo/neo.c
@@ -437,7 +437,7 @@
 int board_late_init(void)
 {
 #ifdef CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG
-	setenv("board_name", board_string());
+	env_set("board_name", board_string());
 #endif
 
 	return 0;