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/board/freescale/mx51evk/mx51evk_video.c b/board/freescale/mx51evk/mx51evk_video.c
index 995fb9a..3715c5d 100644
--- a/board/freescale/mx51evk/mx51evk_video.c
+++ b/board/freescale/mx51evk/mx51evk_video.c
@@ -5,6 +5,7 @@
  */
 
 #include <common.h>
+#include <env.h>
 #include <linux/list.h>
 #include <asm/gpio.h>
 #include <asm/arch/iomux-mx51.h>