factory_install: Change BOARD to take default value from LSB Value.

For devices that want to share same signed factory shim with different
 board name (for downloading different images), none of current board
 selection approach would work, since
  1. It may be not using netboot, so kernel command line cannot be changed
  2. It may need automation so manual select won't help
  3. It may have exactly same hardware so detection script may not help
 So, having the board named defined in LSB file (lsb-release, lsb-factory)
 seems most easy, and is consistent with existing options (board is
 already defined in lsb-release as CHROMEOS_RELEASE_BOARD).

BUG=chrome-os-partner:59376
TEST=1. Set CHROMEOS_RELEASE_BOARD=Locke in lsb-factory of factory install shim
     2. The factory install shim can download image form Locke omaha server.

Change-Id: Ia6dcbed18d8d995a9466af98e5fd68ec78bf41e8
Reviewed-on: https://chromium-review.googlesource.com/297950
Reviewed-by: Chih-Wei Tsai <adam.tsai@quantatw.com>
Tested-by: Chih-Wei Tsai <adam.tsai@quantatw.com>
Reviewed-by: Bowgo Tsai <bowgotsai@chromium.org>
Commit-Queue: Chih-Wei Tsai <adam.tsai@quantatw.com>
Reviewed-on: https://chromium-review.googlesource.com/408027
Commit-Ready: Hung-Te Lin <hungte@chromium.org>
Tested-by: Hung-Te Lin <hungte@chromium.org>
Reviewed-by: Chun-Tsen Kuo <chuntsen@chromium.org>
1 file changed