build: allow forcing building the yocto base

There is no way currently to trigger rebuilding the base if you have
it in overrides/ - there was commented code out that caused the base
to be rebuilt if:

 A) forceComponents=["yocto"] was passed on the command line
 B) Always if it's in overrides/

B) is not tenable considering how slow a build of the base is, but
A) seems useful and harmless, so uncomment it.

https://bugzilla.gnome.org/show_bug.cgi?id=732393