ewaol-config: Put each kas build repository into a common subdirectory

This patch enables a cleaner working directory for the kas build by having each
each repository downloaded to the layers/ subdirectory, and adds this
subdirectory to .gitignore.

Issue-Id: SCM-2619
Signed-off-by: Richard Neill <richard.neill@arm.com>
Signed-off-by: Diego Sueiro <diego.sueiro@arm.com>
Change-Id: I7542127583f2aca0839b50ced318a58611edf83d