cmake: flash: save runner configuration to CMake cache

Persist all the important information needed by the runner package to
the CMake cache. This serves as a parseable record for various tools
which need to understand how to run the binary.

In particular, it will be used by the west tool, which will be
introduced in subsequent patches.

Signed-off-by: Marti Bolivar <marti@opensourcefoundries.com>