detachables: Implement disable dev mode for enterprise

When develper mode is disabled (through FWMP), don't allow user to see
the dev warning screen.  Boot straight to TO_NORM and disable the
cancel option.  Basically, the user will only be able to enable OS
verification, power off, or change the language.  There is also no 30
second timeout during bootup.

BUG=b:65595945
BRANCH=None
TEST=Force disable_dev_boot flag to 1 and ensure the TO_NORM menu is
     displayed w/o a cancel option.  Scroll through options to make
     sure they work as expected.  Make sure debug message is displayed
     indicating dev mode is disabled.  Wait > 30 secs to ensure
     timeout doesn't occur.

Change-Id: I7d2bcd369694e886866f9dedff05d81a40f8270a
Signed-off-by: Shelley Chen <shchen@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/757115
Reviewed-by: Julius Werner <jwerner@chromium.org>
2 files changed