[app] tweak app start to only run apps with entry points defined

Also remove a few empty extraneous app structs that are not doing
anything currently.

app/app.c

@@ -45,6 +45,11 @@ static int app_thread_entry(void *arg) {
 }
 
 static void start_app(const struct app_descriptor *app, bool detach) {
+    /* dont start an app that has no entry point */
+    if (app->entry == NULL) {
+        return;
+    }
+
     uint32_t stack_size = (app->flags & APP_FLAG_CUSTOM_STACK_SIZE) ? app->stack_size : DEFAULT_STACK_SIZE;
 
     printf("starting app %s\n", app->name);

app/stringtests/string_tests.c

@@ -324,7 +324,3 @@ out:
 STATIC_COMMAND_START
 STATIC_COMMAND("string", "memcpy tests", &string_tests)
 STATIC_COMMAND_END(stringtests);
-
-APP_START(stringtests)
-APP_END
-

app/tests/tests.c

@@ -23,12 +23,3 @@ STATIC_COMMAND("fibo", "threaded fibonacci", &fibo)
 STATIC_COMMAND("spinner", "create a spinning thread", &spinner)
 STATIC_COMMAND("cbuf_tests", "test lib/cbuf", &cbuf_tests)
 STATIC_COMMAND_END(tests);
-
-static void tests_init(const struct app_descriptor *app) {
-}
-
-APP_START(tests)
-.init = tests_init,
-.flags = 0,
-APP_END
-