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/aristainetos/aristainetos-v2.c b/board/aristainetos/aristainetos-v2.c
index 6abc215..bdf41b9 100644
--- a/board/aristainetos/aristainetos-v2.c
+++ b/board/aristainetos/aristainetos-v2.c
@@ -670,9 +670,9 @@
 	if (bootmode == 7) {
 		my_bootdelay = getenv("nor_bootdelay");
 		if (my_bootdelay != NULL)
-			setenv("bootdelay", my_bootdelay);
+			env_set("bootdelay", my_bootdelay);
 		else
-			setenv("bootdelay", "-2");
+			env_set("bootdelay", "-2");
 	}
 
 	/* if we have the lg panel, we can initialze it now */