Pack firmware into an image and do update in factory install (separated git).

Since different platforms have different instructions to update their firmware.
In order not to depend on the install shim, we pack all firmware update files
into a single file, i.e. an executable shell script with embedded files.

The original CL (http://codereview.chromium.org/2366001) is reviewed and is
broken down into 5 CLs. This is one of them.

Review URL: http://codereview.chromium.org/2767011