mbv: use new ninja-based build

mbv/apps/helloworld/helloworld.cc

@@ -0,0 +1,26 @@
+#include <cstdint>
+
+struct Gpio {
+    volatile uint32_t data;
+};
+
+#define gpio0 ((Gpio*)0x40000000)
+
+void sleep(int ms) {
+    for (int m = 0; m < ms; m++) {
+        for (int i = 0; i < 10000; i++) {
+            asm volatile ( "" );
+        }
+    }
+}
+
+int main() {
+    int out = 0;
+
+    while (1) {
+        gpio0->data = out;
+        out = (out + 1) % 256;
+
+        sleep(10);
+    }
+}