Add 'clobber' target to Makefile to clean everything.
Add 'clobber' target to Makefile to clean everything.

BUG=none
BRANCH=none
TEST=manual

  make BOARD=link
  make BOARD=snow
  make clobber

Change-Id: I862ec0d0240e48828091259dee7e8aef62544d47
Signed-off-by: Bill Richardson <129945214b1d548d8e49b6c29c43094f8c78057f@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/49370
Reviewed-by: Randall Spangler <62698fdbb84d1779579ee80c3f39fac22017e5bc@chromium.org>

akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,gelraen/cros-ec,alterapraxisptyltd/chromium-ec,longsleep/ec,akappy7/ChromeOS_EC_LED_Diagnostics,fourier49/BZ_DEV_EC,mtk09422/chromiumos-platform-ec,mtk09422/chromiumos-platform-ec,coreboot/chrome-ec,alterapraxisptyltd/chromium-ec,thehobn/ec,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,fourier49/BIZ_EC,longsleep/ec,longsleep/ec,gelraen/cros-ec,eatbyte/chromium-ec,md5555/ec,fourier49/BZ_DEV_EC,fourier49/BIZ_EC,fourier49/BZ_DEV_EC,mtk09422/chromiumos-platform-ec,alterapraxisptyltd/chromium-ec,thehobn/ec,thehobn/ec,gelraen/cros-ec,coreboot/chrome-ec,eatbyte/chromium-ec,gelraen/cros-ec,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,fourier49/BZ_DEV_EC,fourier49/BIZ_EC,eatbyte/chromium-ec,md5555/ec,mtk09422/chromiumos-platform-ec,eatbyte/chromium-ec,longsleep/ec,fourier49/BIZ_EC,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,md5555/ec,alterapraxisptyltd/chromium-ec,thehobn/ec,md5555/ec