owhw: add missing function board_main_top

the board_main_top function was not defined, causing a hard fault
when initializing the board

Change-Id: Ib92003416648822c4115472992850c592bc4047a

firmware/libboard/owhw/source/owhw.c

@@ -21,6 +21,7 @@
 #include "chip.h"
 #include "board.h"
 #include "utils.h"
+#include "usb_buf.h"
 
 static const Pin pins_cardsim[] = PINS_CARDSIM;
 
@@ -42,6 +43,13 @@ void board_exec_dbg_cmd(int ch)
 	}
 }
 
+void board_main_top(void)
+{
+#ifndef APPLICATION_dfu
+	usb_buf_init();
+#endif
+}
+
 void cardsim_set_simpres(uint8_t slot, int present)
 {
 	if (slot > 1)