mbv: use new ninja-based build

mbv/hal/start.cc

@@ -0,0 +1,19 @@
+#include <cstdint>
+
+extern "C" uint32_t _bss_begin, _bss_end, _initial_stack_pointer;
+extern "C" int main();
+
+__attribute__((section(".start"), used, naked))
+void _start() {
+    // clear .bss
+    for (uint32_t* ptr = &_bss_begin; ptr < &_bss_end; ptr++) {
+        *ptr = 0;
+    }
+
+    asm volatile ( "la sp, _initial_stack_pointer" );
+
+    main();
+
+    while (true) {
+    }
+}