Guarantee udev processing is complete on started system-services

Change udev-trigger to be a task job so it needs to complete before the
started event for system-services gets emitted. Update documentation in
system-services to call this out.

BUG=chromium:358213
TEST=System boots up properly.

Change-Id: Ie078526d60b7c869f44e795d8a3c2f3073ddac0e
Reviewed-on: https://chromium-review.googlesource.com/198000
Reviewed-by: Gaurav Shah <[email protected]>
Reviewed-by: Mattias Nissler <[email protected]>
Commit-Queue: Mattias Nissler <[email protected]>
Tested-by: Mattias Nissler <[email protected]>
2 files changed