By cleaning up the code the compiler does not emit the following warning
anymore:

    src/barebox-state.c: In function '__state_string_get':
    src/barebox-state.c:254:6: warning: unused variable 'ret' [-Wunused-variable]
      int ret;
          ^~~
    src/barebox-state.c: In function 'state_get':
    src/barebox-state.c:317:25: warning: unused variable 'v' [-Wunused-variable]
      struct state_variable *v;
                             ^
    src/barebox-state.c:316:14: warning: unused variable 'backend_type' [-Wunused-variable]
      const char *backend_type = NULL;
                  ^~~~~~~~~~~~
    src/barebox-state.c:313:8: warning: unused variable 'path' [-Wunused-variable]
      char *path;
            ^~~~
    src/barebox-state.c: In function 'main':
    src/barebox-state.c:561:9: warning: unused variable 'i' [-Wunused-variable]
         int i;
             ^

Signed-off-by: Ulrich Ölmann <u.oelmann@pengutronix.de>
Signed-off-by: Roland Hieber <rhi@pengutronix.de>