compare_build.sh: Add private directory handling
compare_build.sh: Add private directory handling

This allows compare_build.sh to use private libs
if available.

BRANCH=none
BUG=none
TEST=# Ensure that you have an ec checkout that has a broken
     # public build, but working private build.
     ./compare_build.sh -p
     # This will only work if private was accessible

Signed-off-by: Craig Hesling <383db090fd8ef907a606ac8a9eccd2962bc489cf@chromium.org>
Change-Id: Id5e2d1afe474e0175dbba2cddfd84de7c895a927
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2357870

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