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/el/el6x/el6x.c b/board/el/el6x/el6x.c
index cbe355a..6b98b5c 100644
--- a/board/el/el6x/el6x.c
+++ b/board/el/el6x/el6x.c
@@ -466,7 +466,7 @@
 	add_board_boot_modes(board_boot_modes);
 #endif
 
-	setenv("board_name", BOARD_NAME);
+	env_set("board_name", BOARD_NAME);
 	return 0;
 }