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/net/arp.c b/net/arp.c
index f3ceff9..4c79e09 100644
--- a/net/arp.c
+++ b/net/arp.c
@@ -194,7 +194,7 @@
 		if (net_server_ip.s_addr == net_arp_wait_packet_ip.s_addr) {
 			char buf[20];
 			sprintf(buf, "%pM", &arp->ar_sha);
-			setenv("serveraddr", buf);
+			env_set("serveraddr", buf);
 		}
 #endif