busybox: remove dependency on CLEAN variable

[nalajcie]

CI to pass: phoenix-rtos/phoenix-rtos-project#815

Description

clean is being handled globally (whole build dir is being removed)

Motivation and Context

JIRA: CI-334

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

How Has This Been Tested?

• Already covered by automatic testing.
• New test added: (add PR link here).
• Tested by hand on: (list targets here).

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing linter checks and tests passed.
• My changes generate no new compilation warnings for any of the targets.

Special treatment

• This PR needs additional PRs to work (list the PRs, preferably in merge-order).
• I will merge this PR by myself when appropriate.

[github-actions]

Unit Test Results

2 778 tests   2 508 ✔️  9m 15s ⏱️
   127 suites     270 💤
       1 files           0 ❌

Results for commit 1021ab8.

♻️ This comment has been updated with latest results.