Image Customizer: Operation ordering + scripts refactor.

[cwize1]

1. Only regenerate the initramfs file once during customization, to save time.

2. Regenerate the initramfs file by directly calling mkinitrd instead of reinstalling the initramfs package, to save time and reduce complexity.

3. Remove the existing os.postInstallScripts and os.finalizeImageScripts properties, and replace them with new scripts.postCustomization and scripts.finalizeCustomization properties.

4. Move of the setting SELinux mode to be before the postCustomization scripts.

5. Move the adding of the verity and overlayfs dracut modules to be before the postCustomization scripts.

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Merge Checklist

All boxes should be checked before merging the PR (just tick any boxes which don't apply to this PR)

• The toolchain has been rebuilt successfully (or no changes were made to it)
• The toolchain/worker package manifests are up-to-date
• Any updated packages successfully build (or no packages were changed)
• Packages depending on static components modified in this PR (Golang, *-static subpackages, etc.) have had their Release tag incremented.
• Package tests (%check section) have been verified with RUN_CHECK=y for existing SPEC files, or added to new SPEC files
• All package sources are available
• cgmanifest files are up-to-date and sorted (./cgmanifest.json, ./toolkit/scripts/toolchain/cgmanifest.json, .github/workflows/cgmanifest.json)
• LICENSE-MAP files are up-to-date (./SPECS/LICENSES-AND-NOTICES/data/licenses.json, ./SPECS/LICENSES-AND-NOTICES/LICENSES-MAP.md, ./SPECS/LICENSES-AND-NOTICES/LICENSE-EXCEPTIONS.PHOTON)
• All source files have up-to-date hashes in the *.signatures.json files
• sudo make go-tidy-all and sudo make go-test-coverage pass
• Documentation has been updated to match any changes to the build system
• Ready to merge

Does this affect the toolchain?

NO

Test Methodology

• Manually ran image customizer tool.