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/st/stm32f429-discovery/stm32f429-discovery.c b/board/st/stm32f429-discovery/stm32f429-discovery.c
index 8c8abf6..673e772 100644
--- a/board/st/stm32f429-discovery/stm32f429-discovery.c
+++ b/board/st/stm32f429-discovery/stm32f429-discovery.c
@@ -320,7 +320,7 @@
 		u_id_high = readl(&STM32_U_ID->u_id_high);
 		sprintf(serialno, "%08x%08x%08x",
 			u_id_high, u_id_mid, u_id_low);
-		setenv("serial#", serialno);
+		env_set("serial#", serialno);
 	}
 
 	return 0;