st: Use submodules for system files when possible, use more up to date

files otherwise. Update STM32F4-DISCOVERY demo and board.mk to reflect
changes.

hw/bsp/stm32f407g_disc1/board_stm32f407g_disc1.c

@@ -131,3 +131,10 @@ void HardFault_Handler (void)
 {
   asm("bkpt");
 }
+
+// Required by __libc_init_array in startup code if we are compiling using
+// -nostdlib/-nostartfiles.
+void _init(void)
+{
+
+}