The OpenWrt config needs to have CONFIG_SRC_TREE_OVERRIDE enabled in
order to build from the version checked out in the linked source-tree.
This was forgotten when adding the CI and therefore the CI was always
building the version from the packages feed.
This adds a GitHub actions workflow for building DAWN from this
repository using the OpenWrt SDK.
The resulting package is archived as an artifact available to download.
DAWN currently tries to update it's list of TCP socket endpoints using
mDNS when it is configured to use UDP.
Only update the TCP socket list when DAWN is configured to use TCP
sockets.
Fix a identation mismatch along the way.
