added spi code for linux

linux/UnixFileGuard.h

@@ -0,0 +1,33 @@
+#ifndef LINUX_UTILITY_UNIXFILEGUARD_H_
+#define LINUX_UTILITY_UNIXFILEGUARD_H_
+
+#include <fsfw/returnvalues/HasReturnvaluesIF.h>
+
+#include <string>
+
+#include <fcntl.h>
+#include <unistd.h>
+
+
+class UnixFileGuard {
+public:
+    static constexpr int READ_WRITE_FLAG = O_RDWR;
+    static constexpr int READ_ONLY_FLAG = O_RDONLY;
+    static constexpr int NON_BLOCKING_IO_FLAG = O_NONBLOCK;
+
+    static constexpr ReturnValue_t OPEN_FILE_FAILED = 1;
+
+    UnixFileGuard(std::string device, int* fileDescriptor, int flags,
+            std::string diagnosticPrefix = "");
+
+    virtual~ UnixFileGuard();
+
+    ReturnValue_t getOpenResult() const;
+private:
+    int* fileDescriptor = nullptr;
+    ReturnValue_t openStatus = HasReturnvaluesIF::RETURN_OK;
+};
+
+
+
+#endif /* LINUX_UTILITY_UNIXFILEGUARD_H_ */