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>

api/api.c

@@ -8,6 +8,7 @@
 #include <config.h>
 #include <command.h>
 #include <common.h>
+#include <env.h>
 #include <malloc.h>
 #include <environment.h>
 #include <linux/types.h>