Makefile: Add .config target
Makefile: Add .config target

.config is a file in build/$BOARD that contains all defined config
variables. It is used by external programs to know what features
have been compiled for a specific board.

BUG=chrome-os-partner:33908
TEST=Check .config is generated properly. Check content, check
it updates when new .bin are created.
BRANCH=none

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/233888
Reviewed-by: Vincent Palatin <vpalatin@chromium.org>
(cherry picked from commit fb05f742233c146d8c9c5abb4e55ebd98d990711)
Signed-off-by: Duncan Laurie <70662b43bc1a20b57ded72a2b08be88dd0da4841@chromium.org>

Change-Id: If93c2a393867a9fcb60841e282d8e6b933f3dfec
Reviewed-on: https://chromium-review.googlesource.com/236347
Tested-by: Duncan Laurie <70662b43bc1a20b57ded72a2b08be88dd0da4841@chromium.org>
Trybot-Ready: Duncan Laurie <70662b43bc1a20b57ded72a2b08be88dd0da4841@chromium.org>
Reviewed-by: Duncan Laurie <70662b43bc1a20b57ded72a2b08be88dd0da4841@chromium.org>
Commit-Queue: Duncan Laurie <70662b43bc1a20b57ded72a2b08be88dd0da4841@chromium.org>

thehobn/ec,thehobn/ec,thehobn/ec,thehobn/ec