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/p1_twr/p1_twr.c b/board/freescale/p1_twr/p1_twr.c
index fe99ebf..01cac18 100644
--- a/board/freescale/p1_twr/p1_twr.c
+++ b/board/freescale/p1_twr/p1_twr.c
@@ -5,6 +5,7 @@
 
 #include <common.h>
 #include <command.h>
+#include <env.h>
 #include <hwconfig.h>
 #include <pci.h>
 #include <i2c.h>