env: Move env_get() to env.h

Move env_get() over to the new header file.

Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/bootmenu.c b/cmd/bootmenu.c
index 0ae9d5a..3dc2c85 100644
--- a/cmd/bootmenu.c
+++ b/cmd/bootmenu.c
@@ -6,6 +6,7 @@
 #include <common.h>
 #include <command.h>
 #include <ansi.h>
+#include <env.h>
 #include <menu.h>
 #include <watchdog.h>
 #include <malloc.h>