Linux HAL updates

1. The type correction was merged as part of
   #7 in the
   EIVE project. Quotation of PR

definition of getSpiParameters is `void getSpiParameters(spi::SpiModes& spiMode, uint32_t& spiSpeed, UncommonParameters* parameters = nullptr) const;`.

Here, size_t spiSpeed is passed, which implicitely gets converted to a temporary, which can not be bound to uint32_t& and, at least in gcc 9.3.0, leads to a compiler error.

2. Allow flushing the UART buffers

hal/src/fsfw_hal/linux/uart/UartComIF.h

@@ -41,6 +41,21 @@ public:
     ReturnValue_t readReceivedMessage(CookieIF *cookie, uint8_t **buffer,
             size_t *size) override;
 
+    /**
+     * @brief   This function discards all data received but not read in the UART buffer.
+     */
+    ReturnValue_t flushUartRxBuffer(CookieIF *cookie);
+
+    /**
+     * @brief   This function discards all data in the transmit buffer of the UART driver.
+     */
+    ReturnValue_t flushUartTxBuffer(CookieIF *cookie);
+
+    /**
+     * @brief   This function discards both data in the transmit and receive buffer of the UART.
+     */
+    ReturnValue_t flushUartTxAndRxBuf(CookieIF *cookie);
+
 private:
 
     using UartDeviceFile_t = std::string;