env: Move env_set() to env.h

Move env_set() 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/board/ti/panda/panda.c b/board/ti/panda/panda.c
index f88aa4d..20199da 100644
--- a/board/ti/panda/panda.c
+++ b/board/ti/panda/panda.c
@@ -11,6 +11,7 @@
 #include <asm/arch/clock.h>
 #include <asm/arch/gpio.h>
 #include <asm/gpio.h>
+#include <env.h>
 #include <twl6030.h>
 
 #include "panda_mux_data.h"