Fix the build on ESP targets without board detection and on SDL

[ainyan03]

ESP targets without board detection (ESP32-S2, for example)

• M5GFX.cpp failed to compile: the detection headers and helpers were guarded by the list of supported targets, but init_impl() and autodetect() used them unconditionally ('select_fixed_board' was not declared in this scope, ...).
• A single internal macro now selects the detection path. On the other targets no detection runs and the board stays board_unknown, as in 0.2.33; a fixed_board request is refused with a warning. These targets now also skip NVS reads and writes.

SDL

• setDetectConfig() was defined only inside the ESP section, so calling it failed to link (M5Unified::begin() always calls it).
• It is now defined for every platform. SDL init uses fixed_board when it is set, then M5GFX_BOARD, then the previous default.

The host tests in extras/board_spec that compile excerpts of M5GFX.cpp define the new macro.

Testing

• ESP32-S2 (Arduino 2.x): builds again.
• SDL: a sketch that calls setDetectConfig() and init() links.
• Local build matrix of the versions used by this repository's CI (all supported chips, Arduino and ESP-IDF): all passed, no new warnings. extras/board_spec: npm test and npm run check passed. CI on the fork: passed.