config: add VERSION_FIRMWARE_URL

config: add VERSION_FIRMWARE_URL

This new value points to where firmware can be downloaded. It's not
about a single release but all available firmware releases.

In the next step, this URL should be exposed via `ubus call system
board` as an entry of the `distribution` field. With that value, the
running firmware can check for newer releases.

We already have VERSION_REPO however that's different and only meant for
package managers to download their fitting package indexes/packages.

Signed-off-by: Paul Spooren <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/17780
Signed-off-by: Christian Marangi <[email protected]>