chromeos_startup: add devmode notice to /dev/tty1
chromeos_startup: add devmode notice to /dev/tty1

Let users know they are headed into developer mode.  This is a
precursor to better notification.

TEST=built x86-generic:
     - installed via recovery
     - booted dev mode -> saw warning
     - booted dev mode a second time -> no warning
     - switched to normal mode & booted -> saw notice
     - booted normal after -> no notice

BUG=chrome-os-partner:1357

Change-Id: Iaca96f11cc0a1f15195f2a01ec8c83eb39cd85f0

Review URL: http://codereview.chromium.org/3810004

joshfire/chromiumos-platform-init