core: dumpvar: updates:

Summary:
  - Do not show TARGET_BUILD_APPS if TARGET_BUILD_APPS is unset
    (this one was driving me crazy).

  - Add TARGET_CFX_CLANG_VERSION. Though this is always set, it's only
    used on codefirex build variant. On other variants, it's not useful
    information.

Test Plan: verified

Reviewers: randall.rushing, anders3408, jriordan001, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D175