fix: better RAK15001 detection

[drupol]

This PR improves RAK15001 external flash detection and initialization on RAK4631-based builds, making startup more reliable when a RAK15001 module is present.

Problem

RAK15001 detection could fail during boot on some setups, which prevented external flash from being initialized and path storage limits from being raised accordingly.

What changed

• Added missing power-enable pin definition for RAK4631 board configuration.
• Extended nRF52 early-board init logic to include RAK4631 where needed.
• Switched RAK15001 probing to an explicit WisBlock SPI instance with board-specific pin mapping.
• Explicitly configured WisBlock IO pins to strongly pull WP/HOLD lines high before probing.
• Used the expected WisBlock sensor-slot CS pin during flash initialization.
• Added success logging for detected flash initialization.

Result

• More consistent RAK15001 detection on RAK4631.
• External flash initialization succeeds in scenarios where it previously failed.
• Path store limits are applied when external flash is available.

Validation performed

• Booted firmware with RAK4631 + RAK15001 setup.
• Confirmed successful flash init trace appears at startup.
• Confirmed path store sizing is increased after successful flash initialization.
• Confirmed successful usage locally