add Chapter04 bare example

Chapter04/bare/app/src/main.cpp

@@ -0,0 +1,28 @@
+#include <cstdint>
+
+#include <stm32f0xx_hal.h>
+
+#include <hal.hpp>
+#include <uart_stm32.hpp>
+
+extern "C" int main(void)
+{
+    hal::init();
+
+    hal::uart_stm32 uart(USART2);
+    uart.init();
+
+    uart.puts("Hello world !\r\n");
+
+    std::uint32_t time_prev = hal::time::get_ms();
+    while(1)
+    {
+        if(volatile auto time_now = hal::time::get_ms();
+           time_now - time_prev > 1000)
+        {
+            uart.puts("While loop 1000 ms ping ...\r\n");
+            time_prev = time_now;
+        }
+    }
+}
+