luci-base: break circular luci.config <> luci.model.uci dependency

On certain environments, mainly with the embedded uhttpd interpreter, the
luci.config class cannot be loaded due to a circular dependency with the
luci.model.uci class.

Break up the dependency by deferring the loading of luci.config in
luci.model.uci until it is actually needed.

Fixes #1803, FS#1553.
Signed-off-by: Jo-Philipp Wich <jo@mein.io>