Link the Arduino component automatically when built as an ESP-IDF component

[ainyan03]

Problem

When arduino-esp32 is used as an ESP-IDF component, its -DARDUINO=... definitions are exported as PUBLIC compile options of the Arduino component, so they only reach components that link against it — the application (main) does, this library did not. The public headers of this library change class layout with ARDUINO (for example config_t has serial_baudrate only under ARDUINO, and the display object it embeds changes size), so the application read the same objects with a layout different from the one the library was compiled with: getPanel() returned null and the first drawing call faulted.

The CMakeLists.txt already carried the fix as a commented-out line (list(APPEND COMPONENT_REQUIRES arduino-esp32)), but a user of the component registry cannot enable it.

Change

After register_component(), look for an Arduino component among the components selected for the build (arduino, arduino-esp32 or espressif__arduino-esp32) and link it PUBLIC, so this library is compiled with the same definitions as the application. Only BUILD_COMPONENTS is consulted: a build that does not contain Arduino, or excludes it with COMPONENTS / EXCLUDE_COMPONENTS, is unaffected. Only APIs available since ESP-IDF 4.x are used. This is the same pattern arduino-esp32 itself uses for its optional dependencies (maybe_add_component).

• M5UNIFIED_ARDUINO_COMPONENT=<name> selects a differently named Arduino component; M5UNIFIED_ARDUINO_COMPONENT=OFF disables the automatic dependency.
• Several matching Arduino components in one build is a configuration error (FATAL_ERROR with the variable to set).
• README: note on using the library with Arduino as an ESP-IDF component.

Note for hybrid projects: once Arduino is part of the build, every component that includes this library's headers now compiles them in Arduino mode, which is what the application already did.

Verification

• ESP-IDF 5.5.1 + arduino-esp32 3.3.3 as a component, ESP32: without this change getPanel() is null and the panel is never initialised; with it the display works. Setting M5UNIFIED_ARDUINO_COMPONENT=OFF restores the old behaviour, a custom name and an empty value behave as documented.
• Pure ESP-IDF 5.5.4 and 6.0.2 builds without Arduino: unchanged (no dependency added).
• CI on the ainyan03 fork is green for this branch.