[OE-core] [PATCH] [master, jethro] Fix connectivity check for BB_FETCH_PREMIRRORONLY case

[dl9pf at gmx.de]

Hi!

This patch fixes a bug in the sanity checker that is visible if
you set up a PREMIRROR and start a project from scratch using
BB_FETCH_PREMIRRORONLY set. In this case the sanity check would fail
as BB_NO_NETWORK is not set directly
(just indirectly in the fetcher by BB_FETCH_PREMIRRORONLY).

Reproduce: Set up a PREMIRROR in the local network. Disconnect from
$INTERNET and build a new project using the PREMIRROR on the local net only.

Expected result: Build works using the PREMIRROR

Actual result: Build fails due to CONNECTIVITY_CHECK

Best,
Jan-Simon