cmake: Support specifying Kconfig options on the CLI

This resolves #5723 by adding support for assigning to Kconfig options
from the CMake CLI.

CMake CLI Kconfig options can be specified like so:

'cmake -DCONFIG_ASSERT=y'

The lifetime of such an option is the same as the lifetime of a CMake
cache variable, and in principle any method of populating the
CMakeCache could be used, such as cmake-gui.

This has been implemented by scanning the CMakeCache variables before
Kconfig is executed and writing any that are prefixed with 'CONFIG_'
to a new Kconfig fragment in the build directory that is merged in
with a high precedence.

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