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/htkw/mcx/mcx.c b/board/htkw/mcx/mcx.c
index 1deb2bd..0a02e44 100644
--- a/board/htkw/mcx/mcx.c
+++ b/board/htkw/mcx/mcx.c
@@ -83,7 +83,7 @@
 	if (gpio_get_value(HOT_WATER_BUTTON))
 		return 0;
 
-	setenv("bootcmd", "run swupdate");
+	env_set("bootcmd", "run swupdate");
 
 	return 0;
 }