examples/elf: Extend nxpkg validation coverage.

[aviralgarg05]

Summary

Generate reproducible nxpkg validation fixtures from the examples/elf
hello artifact.

A C host helper creates:

• index.json for a valid package and a mismatched target;
• bad-index.json for a missing artifact;
• pkgtest.nsh for the successful install/list path; and
• pkgfail.nsh for the expected failure path.

The SHA-256 digest and target metadata are derived from the artifact and build
configuration instead of being maintained by hand.

The generated scripts seed both the original /etc/nxpkg/index.json catalog
location and #3642's default /var/lib/nxpkg/index.jsn location. This keeps
the standalone fixtures usable before and after the storage migration.

Impact

• New feature: YES, limited to generated nxpkg validation fixtures.
• User adaptation: NO.
• Build: YES for examples/elf; a C host helper is used by Make and CMake.
• Hardware: NO architecture, board, or driver change.
• Documentation: NO user documentation is required for this validation
  fixture.
• Security: NO runtime trust change.
• Compatibility: The loader and nxpkg runtime behavior are unchanged.
• Other: The valid and failing cases are generated from the same target
  artifact and build metadata.

Testing

Host:

• macOS 26.5, arm64
• xtensa-esp-elf-gcc 14.2.0
  (esp-14.2.0_20241119)

Target:

• esp32s3-xiao:elf
• Test configuration additionally requires
  CONFIG_EXAMPLES_ELF_ROMFS=y and CONFIG_SYSTEM_NXPKG=y. The related
  NuttX branch's current esp32s3-xiao:elf defconfig does not enable
  CONFIG_SYSTEM_NXPKG, so that prerequisite must be added before collecting
  the final target log.

Verification:

• clean Make build;
• repaired CMake loadable-hello target and verified elf_artifacts path;
• all four generated fixture files; and
• full nuttx.bin link.

Testing logs before change:

N/A: the generated validation scripts do not exist on the PR base.

Testing logs after change:

Real hardware (ESP32-S3):
  The complete examples/elf suite finished with end memory delta 0.
  pkgtest.nsh installed hello 1.0.0 and nxpkg list showed its target metadata.
  pkgfail.nsh selected hello-missing, failed artifact acquisition with -2,
  and left the installed list empty.

CMake fixture result:
  hello             1820 bytes
  index.json         413 bytes
  bad-index.json     230 bytes
  pkgtest.nsh        229 bytes
  pkgfail.nsh        234 bytes

  The index SHA-256 exactly matches the CMake-built hello ELF.
  The complete firmware target reached the final nuttx link, where this
  host's Homebrew ARM toolchain lacks nosys.specs. The elf_artifacts target,
  including the generated ROMFS, completed successfully.

The full retained serial transcript should be attached with the signed update.

PR verification Self-Check

• This PR introduces one focused validation-fixture change.
• I have updated all required description fields above.
• I have reviewed and signed the rewritten commit.
• I have collected before/after real-hardware runtime logs.
• This PR adheres to the current contribution and coding guidelines.
• My PR is still work in progress.
• My PR is ready for review and can be safely merged.

[acassis]

@aviralgarg05 please test CMake as well, NuttX used both building systems

[acassis]

@xiaoxiang781216 PTAL

[aviralgarg05]

This PR is ready to merge :)

[linguini1]

This change enforces that the user has Python as part of the build system, which I remember was a contentious issue.

Also, is the elf example not different from the nxpkg application? I don't think we should be generating nxpkg test artifacts from the elf example. It would be better to include those under some test/ directory.

[acassis]

@aviralgarg05 please convert these tools to C, I think it is safer

[aviralgarg05]

I pushed an update for this.

The Python generators added by this PR are gone now. The fixture helper is now a small C host tool, and the CMake path uses the existing in-tree tooling style instead of introducing new Python build steps.

I kept the fixture under examples/elf because it is validating the hello artifact produced by that example itself, but if you still want that split into a dedicated test location, I can do that as a follow-up.

[linguini1]

Squash your commits.

[aviralgarg05]

Updated as one signed commit. The C host generator is shared by the Make and CMake paths, and the CMake graph now defines the loadable hello target and complete fixture outputs. On the ESP32-S3 target, the full ELF suite ended with zero memory delta; the valid package installed and listed, while the missing artifact was rejected without installed state. Fresh CI is queued on the new head.

[aviralgarg05]

One additional scope clarification: the loadable fixture itself now lives under examples/elf/tests, while the host helper is under examples/elf/main/host. The generated nxpkg metadata remains part of this example because it exercises the same ELF loader through the installed-package path; it is test-only output rather than a general package repository feature. A shared package-fixture generator can still be considered separately without coupling this validation PR to that larger design.