Bump CMake minimum version to 3.18

Some CMake files rely on features introduced in 3.18. However, the
minimum required version is currently set to 3.16, which makes some
deployments fail when using 3.16 or 3.17. Fix this by bumping the
required version to 3.18.

Signed-off-by: Balint Dobszay <balint.dobszay@arm.com>
Change-Id: I7263a4ded28e1c6dda1c63c67689b60f087b8182
41 files changed