Add support for merged normal+dev firmware

BUG=chromium-os:17457
TEST=make && make runtests

When this is merged into an actual firmware build, can test it:
* dev switch off -> no dev screen, won't boot self-signed kernel
* dev switch on --> dev warning screen, will boot self-signed kernel
(e.g., it acts like the Cr-48)

Change-Id: I985428256e48b7e05dd4d8fe582a0c0103bf5fb2
Reviewed-on: http://gerrit.chromium.org/gerrit/3901
Reviewed-by: Stefan Reinauer <reinauer@google.com>
Tested-by: Randall Spangler <rspangler@chromium.org>
4 files changed