Skip building Doxygen docs if .nodocs is present
[tests] Use board-mappings.d instead of board-mappings.ini

Directly adding board-mappings.ini is not really supported / useful
anymore, and will be dropped eventually. Move the board-mappings.ini
from the testcases to board-mappings.d which will do the 'merge'
with one file -- but not break when direct board-mappings.ini
installation eventually gets dropped.
[ssu] Move cache files for features/board-mappings to /var/cache