Provide an immutable SAFT startup script.
Provide an immutable SAFT startup script.

SAFT requires specific startup code running after every
reboot, it used to be remounting root fs as writable and
installing a start up script just for that.

With the rootfs verification enabled it became impossible.

The solution is to introduce a startup script which is
always there and checks if a actual SAFT start script
exists on the stateful partition. If it does, this startup
script invokes it passing along the flash device name.

Change-Id: I8e31c12fb58f0fd1558dc69616b9d3f0c5ab0fdb

BUG=chromium-os:6345
TEST=manual

Tested in concert with http://codereview.chromium.org/3297002 (still being
prepared for review)

Review URL: http://codereview.chromium.org/3330005

joshfire/chromiumos-platform-init