Add Python script to generate flash_map C code from devicetree

[Copilot]

The Flash Map subsystem currently generates partition map and flash_area objects using complex nested DT foreach macros. This PR replaces macro-based generation with a Python script that generates explicit C code at compile time.

Changes

New script: scripts/dts/gen_flash_map.py

• Reads EDT pickle to extract fixed-partition and fixed-subpartition nodes
• Generates default_flash_map[] array and individual partition objects (global_fixed_partition_ORD_<N>)
• Uses DT macros in generated code (not computed values) to ensure proper compilation:

  {
      .fa_id = DT_FIXED_PARTITION_ID(DT_N_...),
      .fa_off = FIXED_PARTITION_NODE_OFFSET(DT_N_...),
      .fa_size = DT_REG_SIZE(DT_N_...),
      .fa_dev = DEVICE_DT_GET(DT_MTD_FROM_FIXED_PARTITION(DT_N_...)),
  }

• Supports CONFIG_FLASH_MAP_LABELS via --labels flag

Build integration: subsys/storage/flash_map/CMakeLists.txt

• Invokes gen_flash_map.py during build to generate flash_map_generated.c
• Replaces flash_map_default.c with generated file (unless CONFIG_FLASH_MAP_CUSTOM)
• Regenerates when EDT pickle or script changes

Compatibility

All FIXED_PARTITION_* macros in flash_map.h continue to work unchanged. Generated structures are identical to the macro-based version:

• Sequential partition ID assignment matches gen_defines.py
• Address translation for subpartitions via FIXED_PARTITION_NODE_OFFSET
• Custom flash maps (CONFIG_FLASH_MAP_CUSTOM) unaffected

Benefits

• Replaces nested preprocessor macros with straightforward Python logic
• Generated C code is inspectable for debugging
• Build-time generation with proper dependency tracking

Original prompt

There is Flash Map subsystem that uses partition map generated with use of DT macros and flash_area type objects also generated with use of DT macros.
Propose a script, python, for generating directly C code, replacing the use of DT macros.
The script should be integrated into build system so that the file with flash map and flash area objects gets generated at compile time and compiled in to build.
The change should not break how FIXED_PARTITION_ macros work.

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