Install LuCI for snapshots builds

[Jo-Philipp Wich]

Hi,

> [...]
> Let me explain why:
> Currently the snapshot builders are only building **target-specific**
> packages as well as packages included in the image by default. (We call
> that "phase1"). That means that a single build takes around 2~3 hours,
> depending on the target and the machine carrying out the build. In this
> way we manage to have every target build once approximately every 24
> hours.
> 
> If we wanted to include LuCI, that would basically mean that we will not
> only have to include all the LuCI modules and applications, but also
> **all their dependencies** which is basically half of the packages feed.

This does not sound correct. They would only build those LuCI sub-packages 
being included in the image (luci-base, luci-theme-bootstrap, 
luci-mod-network, luci-mod-system, luci-mod-status, ...) and they mostly 
depend on stuff being present in the default image anyway.

There would be no need at all to build any non-included LuCI package in phase1.

~ Jo