env: Move get_env_id() to env.h

Move this function over to the new header file. Also rename it to have an
env_ prefix like the other functions.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/net/eth_common.c b/net/eth_common.c
index 63f9379..dcc5918 100644
--- a/net/eth_common.c
+++ b/net/eth_common.c
@@ -7,6 +7,7 @@
 
 #include <common.h>
 #include <dm.h>
+#include <env.h>
 #include <environment.h>
 #include <miiphy.h>
 #include <net.h>
@@ -110,7 +111,7 @@
 	static int  env_changed_id;
 	int	env_id;
 
-	env_id = get_env_id();
+	env_id = env_get_id();
 	if ((act == NULL) || (env_changed_id != env_id)) {
 		act = env_get("ethact");
 		env_changed_id = env_id;