Ignore unused private board directories
Ignore unused private board directories

If we're not building a board from a private subdirectory,
there's no need to include all the private build.mk files.

BUG=none
BRANCH=none
TEST=make buildall (with private subdirectories)

Signed-off-by: Bill Richardson <129945214b1d548d8e49b6c29c43094f8c78057f@chromium.org>
Change-Id: I39b61d9b26978702717bf463b47979290cadc8dc
Reviewed-on: https://chromium-review.googlesource.com/344662
Reviewed-by: Dominic Rizzo <a9e5fb9a80871b4edba0a94343af83678d394845@google.com>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec