lighttpd-https-cert: refactor and use uci-defaults fully

This updates Makefile to newer packaging standards as well refactoring it.

Postinst script is dropped in favor of plain uci-defaults. It was implemented in the way that it did one thing if installed to running system and other thing (that was mostly copy) when included in medkit. The replacement uci-default script does the same thing but in more central (deduplicated) way. Uci-default script is run as part of default postinst so it is pretty much the same thing like writing it to postinst.