cmake: Prepend 'cmake_minimum_required()' into 'app' build scripts

Prepend the text 'cmake_minimum_required(VERSION 3.8.2)' into the
application and test build scripts.

Modern versions of CMake will spam users with a deprecation warning
when the toplevel CMakeLists.txt does not specify a CMake
version. This is documented in bug #8355.

To resolve this we include a cmake_minimum_required() line into the
toplevel build scripts. Additionally, cmake_minimum_required is
invoked from within boilerplate.cmake. The highest version will be
enforced.

This patch allows us to afterwards change CMake policy CMP000 from OLD
to NEW which in turn finally rids us of the verbose warning.

The extra boilerplate is considered more acceptable than the verbosity
of the CMP0000 policy.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>