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/syteco/zmx25/zmx25.c b/board/syteco/zmx25/zmx25.c
index bdbf02a..e02dcf2 100644
--- a/board/syteco/zmx25/zmx25.c
+++ b/board/syteco/zmx25/zmx25.c
@@ -156,9 +156,9 @@
 				gpio_set_value(IMX_GPIO_NR(1, 29), 0);
 				gpio_set_value(IMX_GPIO_NR(4, 21), 0);
 
-				setenv("preboot", "run gs_slow_boot");
+				env_set("preboot", "run gs_slow_boot");
 			} else
-				setenv("preboot", "run gs_fast_boot");
+				env_set("preboot", "run gs_fast_boot");
 		}
 	}