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/freescale/common/cmd_esbc_validate.c b/board/freescale/common/cmd_esbc_validate.c
index b06235f..36b620c 100644
--- a/board/freescale/common/cmd_esbc_validate.c
+++ b/board/freescale/common/cmd_esbc_validate.c
@@ -5,6 +5,7 @@
 
 #include <common.h>
 #include <command.h>
+#include <env.h>
 #include <fsl_validate.h>
 
 int do_esbc_halt(cmd_tbl_t *cmdtp, int flag, int argc,