move string helpers from libcros

The only project using these funcs is libcros, so move them here
to avoid having to create a small shared library for it.

While we're at it, rename the files from "string.h" to "cros_string.h"
to avoid implicit collision with the system "string.h".

BUG=chromium-os:29973
TEST=`cbuildbot {x86,amd64,arm}-generic-full` worked

Change-Id: Ic0c5bb481f163f26108c6bcfd18e731b18ff628d
Reviewed-on: https://gerrit.chromium.org/gerrit/25044
Reviewed-by: Ryan Harrison <[email protected]>
Reviewed-by: Chris Masone <[email protected]>
Commit-Ready: Mike Frysinger <[email protected]>
Tested-by: Mike Frysinger <[email protected]>
4 files changed