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/armltd/integrator/integrator.c b/board/armltd/integrator/integrator.c
index c3bafd4..858f74e 100644
--- a/board/armltd/integrator/integrator.c
+++ b/board/armltd/integrator/integrator.c
@@ -116,7 +116,7 @@
 
 int misc_init_r (void)
 {
-	setenv("verify", "n");
+	env_set("verify", "n");
 	return (0);
 }