sysbuild: support Zephyr modules

This commit extends the Zephyr module yaml scheme with additional
entries for sysbuild in the build section.

This allows for Zephyr modules to extend the sysbuild infrastructure
by providing additional CMake and Kconfig files to be included in
sysbuild.

The new settings are:
build:
  sysbuild-cmake: <path>
  sysbuild-kconfig: <path>/<file>
  sysbuild-ext: <true>|<false>
  sysbuild-kconfig-ext:  <true>|<false>

those settings follow the same pattern as the equivalent Zephyr build
settings but are processed by sysbuild.

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>