Define common_inc which includes both public_inc and the project root directory.
The project root directory will allow access to config.h and all the shared/
Replacing all custom '.', '..', '../..', '../shared' etc. include paths with
common_inc reduces clutter in the target definitions and enforces the common
#include directive style, as e.g. including shared/ headers without the
subdirectory name no longer works.
Unfortunately this does not prevent one from using private libweston headers
with the usual include pattern for public headers.
Signed-off-by: Pekka Paalanen <email@example.com>