repo bootstrap: Don't require an initialised repo

Actually it does not make much sense to require the repo to be
initialised, because that counts on the fact that the local config file
exists, which is supposed to be generated using repo bootstrap, so we
have an infinite circle here.

Change-Id: 5f9640d5f0
Story-Id: SF-99
repo init: Mention repo bootstrap

In case the local configuration files is not found, mention that it can
be generated by invoking 'repo bootstrap'.

Story-Id: SF-99
Change-Id: a31c27476e