CHROMIUM: main: prefetch load config files

Forking to pre-fetch certain config files has shown to improve
early boot stages (up to ~200ms on low end devices).

BUG=b:343856616
TEST=CQ + A/B tests of tast bootperf
TEST=`FEATURES=test emerge-$board upstart`

Change-Id: I81d51514ffccbe4259930fbe680abcd9b20c9884
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/upstart/+/5624012
Commit-Queue: Morg <morg@google.com>
Auto-Submit: Morg <morg@google.com>
Reviewed-by: Brian Norris <briannorris@chromium.org>
Reviewed-by: Junichi Uekawa <uekawa@chromium.org>
Tested-by: Morg <morg@google.com>