Moves common iface depencies out of the `if()` statements

Previously, this did not exist, which could cause issues when
building in parallel

subscriber_events was missing a dependency on supervisor_config.

As subscriber_event's dependency on iface was through supervisor_config,
this meant that we could remove it too.

A bunch of difference cmake targets had incorrect dependencies on
iface.

For example, they should be dependending on supervisor_config,
which then dependends on iface.

Compiles libuci in the `cmake --build` compile step,
instead of compiling during the `cmake` configure step,
using ExternalProject.

Compiles `libubox` in the `cmake --build` compile step,
instead of compiling during the `cmake` configure step,
using ExternalProject.

When compiling using the OpenWRT SDK,
libuci is already installed, so there's no need to compile
it manually.