Lack APPLICATION_BINARY_DIR and APPLICATION_SOURCE_DIR

Thanks your porting at first. It help me a lot.

I found a little bug in it. I can't pass overlay config with relative path by -Doverlay_conf in CMake.

For example:

cmake -Bbuild -DBOARD=test -GNinja -DOVERLAY_CONFIG=overlay.conf

cmake will report error log.

CMake Error at cmake/kconfig.cmake:164 (message):
  File not found: /overlay.conf
Call Stack (most recent call first):
  cmake/top.cmake:24 (include)
  CMakeLists.txt:9 (include)

Then I found out the root issue. We lacked variable APPLICATION_BINARY_DIR and APPLICATION_SOURCE_DIR. They are defined in zephyr cmake zephyr/share/zephyr-package/cmake/ZephyrConfig.cmake

I have create a PR to solve this issue: https://github.com/jameswalmsley/cmake-kconfig/pull/2