libchromeos: add API to query /etc/os-release

/etc/os-release fields can come from two different places depending on how we
set them, how the package is installed, etc...
This creates a common API to query field in /etc/os-release and
/etc/os-release.d.

BUG=chromium:420784
TEST=Unittests.

Change-Id: Ic91fd873cd4f6c5e3357991e72d055a12b54c9d1
Reviewed-on: https://chromium-review.googlesource.com/221963
Tested-by: Bertrand Simonnet <bsimonnet@chromium.org>
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Bertrand Simonnet <bsimonnet@chromium.org>
4 files changed