factory_mode: change how reset_required is saved
factory_mode: change how reset_required is saved

Logically OR reset_required with the saved reset_required state when
enabling factory mode, so there is no way to cancel a reset once it's
requested.

BUG=none
BRANCH=cr50
TEST=enable factory mode.

Change-Id: I7a432989b83946570a914f27e806ec2dbc9e9791
Signed-off-by: Mary Ruthven <81380f3403fed74252e92df3ed012a1819ec6296@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1548273
Commit-Ready: ChromeOS CL Exonerator Bot <fa429e5907f00867f797423aea74327bb02c91b6@appspot.gserviceaccount.com>
Reviewed-by: Vadim Bendebury <5515d6d2d0829cbe0dd0dcf2094aaded06d58514@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec