Refactoring EInkDisplay2: Moving from board-specific defines to a unified macro-based configuration

[FoxExe]

I would like to propose a refactoring for EInkDisplay2.cpp and EInkDisplay2.h.
Currently, the file heavily relies on board-specific macro guards (#if defined(MINI_EPAPER_S3), #elif defined(ELECROW_ThinkNode_M5), etc.) to initialize SPI buses and select display drivers.
This approach makes it difficult to maintain, scales poorly, and creates artificial barriers for adding custom DIY targets (for instance, ESP32-S3 boards combined with various third-party E-Paper modules).

The Proposal
Instead of hardcoding specific commercial board names inside the display module core, we could introduce a clean, unified macro configuration pattern. This shifts the configuration load entirely to the variant.h files, making the core display driver generic and future-proof.
We could utilize standard configuration flags like these:

#define USE_EINK
#define DISPLAY_2C           // Two colors (Black/White), default fallback
#define DISPLAY_3C           // Three colors (Red/Black/White or Yellow/Black/White)
#define DISPLAY_SPI 2        // SPI bus index or "HSPI" / "VSPI" macros
#define DISPLAY_ROTATE 90    // Screen rotation in degrees (0, 90, 180, 270)
#define DISPLAY_EN           // [Optional] Display power control pin
#define DISPLAY_EN_INVERTED

Key Enhancements

1. Unified Pin Naming: Transition from prefixing pins with PIN_EINK_* to a generic DISPLAY_* naming scheme (DISPLAY_MOSI, DISPLAY_CS, etc.). This aligns with potential future refactoring where common drawing routines can check the abstract display type (OLED, EINK, TFT), rather than maintaining decoupled modules
2. Flexible SPI Routing: DIY builds often isolate the E-Paper display on a separate hardware SPI bus to avoid bus contention with LoRa transceivers. Explicitly defining DISPLAY_SPI simplifies routing the display onto an independent bus instance.

Example Implementation

// Connect to the display using universal macro settings
bool EInkDisplay::connect() {
	LOG_INFO("Do EInk init");

	// Enable display power rail if DISPLAY_EN pin is specified
#ifdef DISPLAY_EN
	if (DISPLAY_EN != -1) {
		pinMode(DISPLAY_EN, OUTPUT);
#ifdef DISPLAY_EN_INVERTED
		digitalWrite(DISPLAY_EN, HIGH);
#else
		digitalWrite(DISPLAY_EN, LOW);
#endif
		delay(10);
	}
#endif

	// Configure SPI bus interface based on variant macros
#if defined(DISPLAY_SPI) && (DISPLAY_SPI > 1)
	// Instantiate a custom hardware SPI bus on the fly using the provided number
	spi_bus = new SPIClass(DISPLAY_SPI);
#else
	// Default fallback to the standard global Arduino SPI instance
	spi_bus = &SPI;
#endif

	// Assign custom pins if they are fully defined in the variant header
#if defined(DISPLAY_SCLK) && defined(DISPLAY_MOSI) && defined(DISPLAY_CS)
	spi_bus->begin(DISPLAY_SCLK, -1, DISPLAY_MOSI, DISPLAY_CS);
#else
	// Fall back to the architecture's default hardware pins for this bus
	spi_bus->begin();
#endif

	// Instantiate low-level panel driver
	auto lowLevel = new EINK_DISPLAY_MODEL(DISPLAY_CS, DISPLAY_DC, DISPLAY_RES, DISPLAY_BUSY);
	adafruitDisplay = new GxEPD2_BW<EINK_DISPLAY_MODEL, EINK_DISPLAY_MODEL::HEIGHT>(*lowLevel);
	adafruitDisplay->init(115200, true, 20, false, *spi_bus, SPISettings(4000000, MSBFIRST, SPI_MODE0));

	// 4. Apply screen rotation (0, 90, 180, 270 degrees)
#if defined(DISPLAY_ROTATE)
#if DISPLAY_ROTATE == 90
	adafruitDisplay->setRotation(1);
#elif DISPLAY_ROTATE == 180
	adafruitDisplay->setRotation(2);
#elif DISPLAY_ROTATE == 270
	adafruitDisplay->setRotation(3);
#else
	adafruitDisplay->setRotation(0);
#endif
#else
	adafruitDisplay->setRotation(1); // Default landscape orientation
#endif

	// Set full screen update window
	adafruitDisplay->setPartialWindow(0, 0, displayWidth, displayHeight);

	return true;
}

Seamless Backward Compatibility:
To avoid breaking existing board targets and prevent the need to modify dozens of current variant.h files simultaneously, we can implement temporary macro mapping inside EInkDisplay2.h.
If the legacy PIN_EINK_* definitions are detected, they will automatically map to the new unified DISPLAY_* macros. This allows a smooth, phased migration across the codebase.

// Fallback for legacy target configurations
#if defined(PIN_EINK_BUSY) && !defined(DISPLAY_BUSY)
	#define DISPLAY_BUSY PIN_EINK_BUSY
#endif
#if defined(PIN_EINK_RES) && !defined(DISPLAY_RES)
	#define DISPLAY_RES  PIN_EINK_RES
#endif
#if defined(PIN_EINK_DC) && !defined(DISPLAY_DC)
	#define DISPLAY_DC   PIN_EINK_DC
#endif
#if defined(PIN_EINK_CS) && !defined(DISPLAY_CS)
	#define DISPLAY_CS   PIN_EINK_CS
#endif
#if defined(PIN_EINK_SCLK) && !defined(DISPLAY_SCLK)
	#define DISPLAY_SCLK PIN_EINK_SCLK
#endif
#if defined(PIN_EINK_MOSI) && !defined(DISPLAY_MOSI)
	#define DISPLAY_MOSI PIN_EINK_MOSI
#endif
#if defined(PIN_EINK_EN) && !defined(DISPLAY_EN)
	#define DISPLAY_EN   PIN_EINK_EN
#endif

// Map older boards using HSPI to the new unified SPI bus configuration
#if !defined(DISPLAY_SPI) && (defined(MINI_EPAPER_S3) || defined(ELECROW_ThinkNode_M5) || defined(TLORA_T3S3_EPAPER))
	#define DISPLAY_SPI  2
#endif