env: Move env_set_hex() to env.h

Move env_set_hex() over to the new header file along with env_set_addr()
which uses it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/board/xilinx/zynqmp/cmds.c b/board/xilinx/zynqmp/cmds.c
index 321670d..ed7ba58 100644
--- a/board/xilinx/zynqmp/cmds.c
+++ b/board/xilinx/zynqmp/cmds.c
@@ -5,6 +5,7 @@
  */
 
 #include <common.h>
+#include <env.h>
 #include <malloc.h>
 #include <asm/arch/hardware.h>
 #include <asm/arch/sys_proto.h>