env: Rename getenv/_f() to env_get()

We are now using an env_ prefix for environment functions. Rename these
two functions for consistency. Also add function comments in common.h.

Quite a few places use getenv() in a condition context, provoking a
warning from checkpatch. These are fixed up in this patch also.

Suggested-by: Wolfgang Denk <wd@denx.de>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/sunxi/board.c b/board/sunxi/board.c
index 26732dc..70e0143 100644
--- a/board/sunxi/board.c
+++ b/board/sunxi/board.c
@@ -602,7 +602,7 @@
 	char *serial_string;
 	unsigned long long serial;
 
-	serial_string = getenv("serial#");
+	serial_string = env_get("serial#");
 
 	if (serial_string) {
 		serial = simple_strtoull(serial_string, NULL, 16);
@@ -694,7 +694,7 @@
 			else
 				sprintf(ethaddr, "eth%daddr", i);
 
-			if (getenv(ethaddr))
+			if (env_get(ethaddr))
 				continue;
 
 			/* Non OUI / registered MAC address */
@@ -708,7 +708,7 @@
 			eth_env_set_enetaddr(ethaddr, mac_addr);
 		}
 
-		if (!getenv("serial#")) {
+		if (!env_get("serial#")) {
 			snprintf(serial_string, sizeof(serial_string),
 				"%08x%08x", sid[0], sid[3]);