H8I8O (#209)

* Add defines for LED, button and serial port

To make supporting different board layouts easier, this introduces some new defines:

For all platforms:
- `KNX_LED` -  the programming LED, defaults to `LED_BUILTIN`
- `KNX_BUTTON` - the programming button, defaults to `0`

For Arduino platforms:
- `KNX_DEBUG_SERIAL` - the serial port that is used for debugging, defaults to `Serial`
- `KNX_SERIAL` - the serial port used for communication with the TPUART, default depends on platform

* GD32 flash workaround

Addressing #181

At least some GD32 devices seem to require unlocking the flash twice. As the unlock function exits with `HAL_OK` if the flash is already unlocked, STM32 devices can run the same code without issues.

* Add H8I8O and H8C09 configurations to knx-demo

Addressing #181

These configurations both serve as examples for the use of these boards and other custom boards that don't use the standard pinout.

* Add STM32 unlock target

This copies the brute force unlock approach by @pavkriz into a custom target. With this commit, it is possible to automatically unlock e.g. a H8I8O board without leaving the PlatformIO IDE.

See #181

src/stm32_platform.cpp

@@ -4,9 +4,13 @@
 #include <EEPROM.h>
 #include "knx/bits.h"
 
+#ifndef KNX_SERIAL
+#define KNX_SERIAL Serial2
+#endif
+
 Stm32Platform::Stm32Platform()
 #ifndef KNX_NO_DEFAULT_UART
-    : ArduinoPlatform(&Serial2)
+    : ArduinoPlatform(&KNX_SERIAL)
 #endif
 {
 }
@@ -44,7 +48,7 @@ uint8_t * Stm32Platform::getEepromBuffer(uint16_t size)
         _eepromPtr = new uint8_t[size];
         eeprom_buffer_fill();
         for (uint16_t i = 0; i < size; ++i)
-            _eepromPtr[i] = eeprom_buffered_read_byte(i);   
+            _eepromPtr[i] = eeprom_buffered_read_byte(i);
     }
     
     return _eepromPtr;
@@ -56,6 +60,11 @@ void Stm32Platform::commitToEeprom()
         return;
     for (uint16_t i = 0; i < _eepromSize; ++i)
         eeprom_buffered_write_byte(i, _eepromPtr[i]);
+    // For some GD32 chips, the flash needs to be unlocked twice
+    // and the first call will fail. If the first call is
+    // successful, the second one (inside eeprom_buffer_flush)
+    // does nothing.
+    HAL_FLASH_Unlock();
     eeprom_buffer_flush();
 }