xbuddy: Expose alias lookup method.

This is needed for applications (e.g. cros flash) to be able to properly
map image aliases (e.g. 'project_sdk') into the correct XBuddy path.
The signature is slightly changed by making the board and version
arguments optional, deferring to pre-initialized board/version.

TEST=Unit test

Change-Id: I5fa5ad77eff7768e078f842f8c9a14c369535fca
Reviewed-on: https://chromium-review.googlesource.com/267125
Trybot-Ready: Gilad Arnold <garnold@chromium.org>
Tested-by: Gilad Arnold <garnold@chromium.org>
Reviewed-by: Chris Sosa <sosa@chromium.org>
Commit-Queue: Gilad Arnold <garnold@chromium.org>
2 files changed