Add newest shellmatta and include missing syscalls

stm-firmware/syscalls.c

@@ -91,3 +91,14 @@ int _write(int fd, const void *buf, int count)
 		uart_send_array_with_dma(&shell_uart, (char *)buf, count);
 	return count;
 }
+
+int _getpid()
+{
+	return 0;
+}
+
+int _kill(int pid)
+{
+	return -1;
+}
+