Specify paths to "X.org" configs in overlays

Since https://crrev.com/c/1535563 moved configuration files for
end-of-life boards out of platform/xorg-conf into overlays, touch tests
for those boards have been failing. We want to
keep those tests around, as they check edge cases that might not be
covered elsewhere. This CL adds a "conf_directory" property that
specifies the path to the relevant overlay directory.

Also removed support for the "identifier" property, as it's been
replaced by "identifiers" in all property files.

BUG=none
TEST=run touch tests, check "No such file or directory" errors go away
     for affected boards (some tests are still broken for other reasons)

Change-Id: I7c5676f59b9520a090c56ca6bb525a266fce7775
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/mttools/+/2138711
Tested-by: Harry Cutts <hcutts@chromium.org>
Reviewed-by: Sean O'Brien <seobrien@chromium.org>
Commit-Queue: Harry Cutts <hcutts@chromium.org>
13 files changed