doc: fix Kconfig.shield sourcing build issue

Fix documentation build issue where generating the kconfig rST was
failing due to missing Kconfig.shield* files. Add an extra step in
CMakeLists.txt to properly generate them, similarly to what is already
done for Kconfig.soc*.

Signed-off-by: Fabio Utzig <fabio.utzig@nordicsemi.no>
1 file changed