chromeos_startup: add env for firmware updater to decide its action
chromeos_startup: add env for firmware updater to decide its action

The firmware updater in boot stage may follow some special logic,
like allowing reboot for EC updates.
To make a more descriptive hint for updater, the environment
variable as "IS_CHROMEOS_STARTUP=1" should be passed into updater.

Ref: http://codereview.chromium.org/4006007

BUG=chrome-os-partner:1495
TEST=trying booting on a device and touched $NEED_FIRMWARE_UPDATE,
     then see the updater running fine.

Change-Id: I36f54d5b98bf9413b41b21323b8a01bbbbac804b

Review URL: http://codereview.chromium.org/4173001

joshfire/chromiumos-platform-init