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>

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>