Remove "Enable Developer Mode" entry in Recovery Menu when board is
already in developer mode.  Having the entry present was confusing
users trying to enable dev mode when machine already in dev mode and
seeing nothing happen.  Changing VbExDisplayMenu API so that a
disabled_idx_mask is passed down to allow for the general
disabling/enabling of menu items.

BUG=b:63078243, b:35585623
BRANCH=None
TEST=reboot into recovery with DUT already in dev mode.  Make sure
     Enable Developer Mode entry is not displayed.
     reboot into recovery with DUT in normal mode.  Make sure
     Enable Developer Mode entry is displayed and can be selected.
CQ-DEPEND=CL:598430

Change-Id: I70f07acd740169e077d19148f70e66f9c78bd520
Signed-off-by: Shelley Chen <shchen@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/565335


Reviewed-by: Julius Werner <jwerner@chromium.org>