build: Use only one Makefile for apps

- Use one common Makefile for all test device apps.
- Use a single copy of syscall.[Sh].
- Update docs for building.

hw/application_fpga/apps/defaultapp/main.c

@@ -1,13 +1,12 @@
 // Copyright (C) 2025 - Tillitis AB
 // SPDX-License-Identifier: GPL-2.0-only
 
+#include <fw/tk1/reset.h>
+#include <fw/tk1/syscall_num.h>
+#include <syscall.h>
 #include <tkey/debug.h>
 #include <tkey/led.h>
 
-#include "../testapp/syscall.h"
-#include "../tk1/reset.h"
-#include "../tk1/syscall_num.h"
-
 int main(void)
 {
 	struct reset rst = {0};