Added support for boards with only private overlays. (crosutils)

Added support for boards with only private overlays. (crosutils)

If a board has a public overlay it will be used as the primary overlay. However, with this change, if a board only has a private overlay it will be used as the primary.

BUG=7339
TEST=Ran setup_board, build_packages, and build_image on a board with only a private overlay (locally created). Ran setup_board, build_packages, and build_image on a board with both public and private overlays.

Change-Id: Ic56e4951272ddb80511a213e3a2e54419267a49a

Related Issues:
http://codereview.chromium.org/3544009
http://codereview.chromium.org/3571015

Committed: http://chrome-svn/viewvc/chromeos?view=rev&revision=7efdebf

[Dave Parker, 2010-10-05 20:38:31]

Related Issues: 
http://codereview.chromium.org/3544009 
http://codereview.chromium.org/3571015

[Dave Parker, 2010-10-06 23:20:36]

I've centralized logic for selecting a board's primary overlay into
cros_overlay_list (per feedback).

Note: I did not update parallel_emerge as I didn't want to call out to a shell
script.

Related CL: http://codereview.chromium.org/3571015

[robotboy, 2010-10-11 17:53:44]

cros_overlay_list: LGTM

Thanks.

[Dave Parker, 2010-12-01 00:55:36]

I've made an additional change to cros_overlay_list to support boards that have
private variant overlays with no corresponding non-private variant overlay.

[robotboy, 2010-12-01 01:41:54]

LGTM, thanks.

I tested by manually running to verify that the correct overlays will be
included for Seaboard and Dev Board.