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/tqc/tqma6/tqma6.c b/board/tqc/tqma6/tqma6.c
index fcdea34..14991fd 100644
--- a/board/tqc/tqma6/tqma6.c
+++ b/board/tqc/tqma6/tqma6.c
@@ -251,7 +251,7 @@
 
 int board_late_init(void)
 {
-	setenv("board_name", tqma6_get_boardname());
+	env_set("board_name", tqma6_get_boardname());
 
 	tqma6_bb_board_late_init();