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/spl.c

@@ -7,6 +7,7 @@
 #include <common.h>
 #include <command.h>
 #include <cmd_spl.h>
+#include <env.h>
 #include <linux/libfdt.h>
 
 DECLARE_GLOBAL_DATA_PTR;