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/include/syscall.h

@@ -0,0 +1,11 @@
+// SPDX-FileCopyrightText: 2024 Tillitis AB <tillitis.se>
+// SPDX-License-Identifier: BSD-2-Clause
+
+#include <stdint.h>
+
+#ifndef TKEY_APP_SYSCALL_H
+#define TKEY_APP_SYSCALL_H
+
+int syscall(uint32_t number, uint32_t arg1, uint32_t arg2, uint32_t arg3);
+
+#endif