toaster: update toasterconf.json for fido release

toaster: update toasterconf.json for fido release

We update the toasterconf.json for the 1.8 "fido"
release.

A small update to the release help text in the
toasterconf.json file shipped with the
openembedded-core layer.

We now make explicit that Toaster will build with
the tip of the selected branch, and we add links
to the OpenEmbedded repository.

Signed-off-by: Belen Barros Pena <belen.barros.pena@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>