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>

cmd/reiser.c

@@ -11,6 +11,7 @@
 #include <common.h>
 #include <config.h>
 #include <command.h>
+#include <env.h>
 #include <image.h>
 #include <linux/ctype.h>
 #include <asm/byteorder.h>