this allows targets to use the new uci-default helper which will generate
a file called /etc/board.json. a tool called /bin/config_generate can then
be used to generate the default uci settings.

Signed-off-by: John Crispin <blogic@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42185 3c298f89-4303-0410-b956-a3cf2f4a3e73