Remove the last remnants of board-specific headers
Remove the last remnants of board-specific headers

Commit d57248cd528ad4513caeb3800570e98741f8cdb6 removed board-specific
headers, but c.mk still contains the preprocessor command-line
definitions to support them.  Drop those too.

BUG=None
TEST=Build adhd

Change-Id: I74d305c746741a88d985cece3bfc4607c334b8c1
Reviewed-on: https://gerrit.chromium.org/gerrit/28604
Tested-by: Josh Triplett <c028c213ed5efcf30c3f4fc7361dbde0c893c5b7@joshtriplett.org>
Reviewed-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>
Commit-Ready: Josh Triplett <c028c213ed5efcf30c3f4fc7361dbde0c893c5b7@joshtriplett.org>

drinkcat/adhd,drinkcat/adhd,drinkcat/adhd,drinkcat/adhd,drinkcat/adhd,drinkcat/adhd