Refine the method to find pch and pcm files.

Refine the method to find pch and pcm files.

When compile user kernels, we need to find the precompiled header
file and the precompiled module file. The previous implementation
will find the build directory then find the system directory.

This is not elegant when it is packaged to a distro. It doesn't
need to search the build directory. So I change the default search
path to the system directory only. And for the deveoper, I change
the build script to set a proper environment variable and make the
gbe bin generator and the utest could find the local pch files and
pcm files firstly.

The only change is now, after the build process. Before the user
run the utests, it need to set up the environment firstly. Just
invoke

. utest/setenv.sh.

Then everything should be the same as previous. This setenv.sh also
set the OCL_KERNEL_PATH, so you don't need to set it manually now.

This patch also update the document.

v2:
add the missing setenv.sh.

Signed-off-by: Zhigang Gong <zhigang.gong@intel.com>
Tested-by: "Song, Ruiling" <ruiling.song@intel.com>