Fix permission in crosh-usb/update_firmware
Fix permission in crosh-usb/update_firmware

Since crosh is executed with 'cronos' account,
we need to grant root permission when running firmware update
(NOTE: chromeos-install, on the contrary, needs using cronos and then it will use 'sudo' to get root privileges )

BUG=chromium-os:6452
TEST=manual:
1. create a ChromeOS image with fimrware updates on USB and boot with it
2. enter crosh (ctrl-alt-T) and execute: update_firmware
3. Expected Result:
   firmware update successfully after some progress/status messages

Change-Id: Id78610a7040ccacf2f3b439dbfdabf56ef621a86

Review URL: http://codereview.chromium.org/3347014

joshfire/chromiumos-platform-window_manager,joshfire/chromiumos-platform-window_manager,joshfire/chromiumos-platform-window_manager,joshfire/chromiumos-platform-window_manager