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/nokia/rx51/rx51.c b/board/nokia/rx51/rx51.c
index 6a4427a..889e748 100644
--- a/board/nokia/rx51/rx51.c
+++ b/board/nokia/rx51/rx51.c
@@ -413,7 +413,7 @@
 
 	/* set env variable attkernaddr for relocated kernel */
 	sprintf(buf, "%#x", KERNEL_ADDRESS);
-	setenv("attkernaddr", buf);
+	env_set("attkernaddr", buf);
 
 	/* initialize omap tags */
 	init_omap_tags();