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/BuR/common/common.c b/board/BuR/common/common.c
index 89087d7..148fc90 100644
--- a/board/BuR/common/common.c
+++ b/board/BuR/common/common.c
@@ -10,6 +10,7 @@
  */
 #include <version.h>
 #include <common.h>
+#include <env.h>
 #include <fdtdec.h>
 #include <i2c.h>
 #include <lcd.h>