ewaol-config/kas: Only include meta-arm layers when needed

Since in ewaol-release.yml we pin meta-arm repo, we exclude its layers in
ewaol-base.yml and eneable them in arm-machines.yml when included by specific
kas configs (e.g. n1sdp.yml).

Issue-Id: SCM-3553
Signed-off-by: Diego Sueiro <diego.sueiro@arm.com>
Change-Id: I1aeb60c0ce660cde6af31de8393905d106b6cf9c