I re-factored the extend call into its own module, since it doesn't necessarily
need to be couple with rollbackfirmwaresetup.
BUG=chrome-os-partner:2372
TEST=Tried emerge-x86-generic and emerge-arm-generic vboot_reference.
Change-Id: I0d3115b10b686133e63e304570325ebdbd50ae3a
Review URL: http://codereview.chromium.org/6696006