ewaol-config/kas: Set default number of threads inside ewaol-base.yml config

This patch fix the bug with using kas directly with ewaol-config files,
where 'BB_NUMBER_THREADS' variable was unset, resulting bitbake to use only
single core and long build time.

Issue-Id: SCM-3553
Signed-off-by: Kamil Dziezyk <kamil.dziezyk@arm.com>
Change-Id: I2eadeb9539eb8682633be35f515f530d366ae2a8