env: Move env_init() to env.h

Move env_init() over to the new header file.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/board/ti/am57xx/board.c b/board/ti/am57xx/board.c
index 1a903f1..0dacfd4 100644
--- a/board/ti/am57xx/board.c
+++ b/board/ti/am57xx/board.c
@@ -8,6 +8,7 @@
  */
 
 #include <common.h>
+#include <env.h>
 #include <palmas.h>
 #include <sata.h>
 #include <usb.h>