Fix honoring NACL_DEBUG envrionment variable.
Fix honoring NACL_DEBUG envrionment variable.

A recent change to the handling of the --debug command
line flag broke this.  This broke the sdk_bundle builder
which was setting NACL_DEBUG.

When --debug is not specified we want to fall back to
checking the NACL_DEBUG environment variable.

BUG= https://code.google.com/p/naclports/issues/detail?id=197
R=binji@chromium.org

Review URL: https://codereview.chromium.org/1008033002

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