tools/kas-ci-build: Add '-j' and '--out-dir' parameters to build script

This patch:
 (*) adds '-j' parameter to set maximum number of cpu threads available for
     bitbake.
 (*) adds '--out-dir' parameter to allow user to change build directory.
 (*) renames 'layer_dir' variable into 'project_root', which is more accurate.
 (*) updates documentation.

Issue-Id: SCM-2620
Signed-off-by: Kamil Dziezyk <kamil.dziezyk@arm.com>
Change-Id: Ia4d12550e9c2b982147f3b537b56ff73c7cb0121