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/ti/am43xx/board.c b/board/ti/am43xx/board.c
index 933596d..55098bf 100644
--- a/board/ti/am43xx/board.c
+++ b/board/ti/am43xx/board.c
@@ -626,7 +626,7 @@
 	 * on HS devices.
 	 */
 	if (get_device_type() == HS_DEVICE)
-		setenv("boot_fit", "1");
+		env_set("boot_fit", "1");
 #endif
 	return 0;
 }