Remove gen-collateral when rebuilding

[joonho3020]

Related PRs / Issues:

This PR addresses #1341.
Removes $(OUT_DIR) which is gen-collateral when re-compiling.

Type of change:

• Bug fix
• New feature
• Other enhancement

Impact:

• RTL change
• Software change (RISC-V software)
• Build system change
• Other

Contributor Checklist:

• Did you set main as the base branch?
• Is this PR's title suitable for inclusion in the changelog and have you added a changelog:<topic> label?
• Did you state the type-of-change/impact?
• Did you delete any extraneous prints/debugging code?
• Did you mark the PR with a changelog: label?
• (If applicable) Did you update the conda .conda-lock.yml file if you updated the conda requirements file?
• (If applicable) Did you add documentation for the feature?
• (If applicable) Did you add a test demonstrating the PR?

• (If applicable) Did you mark the PR as Please Backport?

[abejgonzalez]

Why not just change the FIRTOOL recipe to rm -rf $(OUT_DIR) and list it as a target? That seems less invasive.

[jerryz123]

In the process, can we also rename OUT_DIR to something more descriptive? GEN_COLLATERAL_DIR?

[jerryz123]

Implementation looks clean. I'll test this once this gets merged.

[abejgonzalez]

The prior approach didn't enforce that the collateral directory would be rebuilt. This new approach basically forces sim_files to be generated after the FIRTOOL target (this was already done with the sim_common_files target) and removes the collateral directory in the FIRTOOL target.

[abejgonzalez]

The renaming LGTM. Pending reviews from others on the rm -rf