add defaults to optional vars in _targets/build.common

[xvuko]

Motivation and Context

This is needed to enable nounset option in build.sh: phoenix-rtos/phoenix-rtos-build#184

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)
• Chore (refactoring, style fixes, git/CI config, submodule management, no code logic changes)

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

7 460 tests  ±0   6 745 ✅ ±0   38m 59s ⏱️ - 1m 13s
  420 suites ±0     715 💤 ±0 
    1 files   ±0       0 ❌ ±0 

Results for commit fa03b50. ± Comparison against base commit 6f87c3f.

♻️ This comment has been updated with latest results.

[anglov]

I'am ok with current state, but needs @nalajcie approval

I think this one is safe enabling nounset may be more controversial.