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/birdland/bav335x/board.c b/board/birdland/bav335x/board.c
index b49365e..c512225 100644
--- a/board/birdland/bav335x/board.c
+++ b/board/birdland/bav335x/board.c
@@ -301,8 +301,8 @@
 int board_late_init(void)
 {
 #ifdef CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG
-	setenv("board_name", "BAV335xB");
-	setenv("board_rev", "B"); /* Fix me, but why bother.. */
+	env_set("board_name", "BAV335xB");
+	env_set("board_rev", "B"); /* Fix me, but why bother.. */
 #endif
 	return 0;
 }