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/am65x/evm.c b/board/ti/am65x/evm.c
index 7bd8c4f..e01adcd 100644
--- a/board/ti/am65x/evm.c
+++ b/board/ti/am65x/evm.c
@@ -14,6 +14,7 @@
 #include <asm/gpio.h>
 #include <asm/io.h>
 #include <asm/omap_common.h>
+#include <env.h>
 #include <spl.h>
 #include <asm/arch/sys_proto.h>