libweave: Partially copy string_utils from libchromeos To move dependency on libchromeos. BUG=brillo:1257 TEST=`FEATURES=test emerge-gizmo libweave buffet Change-Id: Ie9e4bb1a754373c26ed6497cdb3ab03df8a78d87 Reviewed-on: https://chromium-review.googlesource.com/293613 Reviewed-by: Vitaly Buka <vitalybuka@chromium.org> Commit-Queue: Vitaly Buka <vitalybuka@chromium.org> Tested-by: Vitaly Buka <vitalybuka@chromium.org>