Improve the discovery of db.h

Originally includes was searched in sys includes and
/usr/local/BerkeleyDB.4.8/include. On systems with multiple slots and
with default set to something different than db:4.8 this cause using of
wrong includes.
To avoid that use the same find technique as for libraries.

Open-webOS-DCO-1.0-Signed-off-by: Nikolay Orliuk <norluk@luxoft.com>
Change-Id: I435bbdf80cd2493bf0df962a917700b362d24263