Add --vmargs-path option to support custom vm.args file paths

[Copilot]

This PR adds a new --vmargs-path option to erlinit that allows users to specify a custom path to the vm.args file. This complements the existing --boot option and provides flexibility for deployments that need to use vm.args files in non-standard locations.

Changes

• Added --vmargs-path command-line option that accepts a path to a vm.args file
• Supports both absolute paths (e.g., /data/vm.args) and relative paths (e.g., custom.vm.args relative to the release version directory)
• If the specified file exists, erlinit uses it
• If the specified file doesn't exist, erlinit prints a warning and falls back to the default vm.args location in the release version directory

Implementation

The implementation follows the same pattern as the existing --boot option for consistency:

1. Check if user specified a custom vmargs path
2. Resolve absolute vs relative paths
3. Verify the file exists
4. Fall back to default location if not found, with a warning

Example Usage

In erlinit.config:

--vmargs-path
/data/custom.vm.args

Or on the command line:

erlinit --vmargs-path custom.vm.args

Testing

Added three comprehensive test cases:

• 068_vmargs_path: Verifies a valid custom vm.args path (relative) works correctly
• 069_vmargs_path_missing: Verifies fallback behavior when specified file doesn't exist
• 070_vmargs_path_absolute: Verifies absolute path support

All 70 tests pass successfully.

Fixes #[issue_number]

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add an erlinit.conf option to support passing a path for the vm.args</issue_title> <issue_description>When the option is passed, erlinit should check that the file exists. If it does, it should use it. If it doesn't, it should print a warning and find the OTP release's vm.args instead.</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes nerves-project/erlinit#130

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