Use official JSON database

[mwarning]

Just a heads up.

OpenWrt can now create JSON files that can serve as a database for firmware selectors:
openwrt/openwrt@881ed09
(Then use Global build settings ---> [*] Create JSON info files per build image.)

I locally cherry-picked it for the the openwrt-19.07 branch and the JSON files were produced without problems (but vendor, model and variant are merged - that's no problem).

Here is an example: https://github.com/mwarning/yet_another_firmware_selector