build private files if present

If we have a private/ directory, check the build.mk there and build the
content, else safely ignore that part of the build.

Signed-off-by: Vincent Palatin <vpalatin@chromium.org>

BUG=None
TEST="make BOARD=link" with and without a private directory containing
some code with new console commands. Run the firmware and check if the console
commands are actually present.

Change-Id: I690ed97be24d029628e4acf508299dcbab657100
1 file changed