configure: Remove --source-path option

Normally configure identifies the source path by looking
at the location where the configure script itself exists.
We also provide a --source-path option which lets the user
manually override this.

There isn't really an obvious use case for the --source-path
option, and in commit 92712822 in 2017 we
accidentally added some logic that looks at $source_path
before the command line option that overrides it has been
processed.

The fact that nobody complained suggests that there isn't
any use of this option and we aren't testing it either;
remove it. This allows us to move the "make $source_path
absolute" logic up so that there is no window in the script
where $source_path is set but not yet absolute.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-id: 20190318134019.23729-1-peter.maydell@linaro.org