metadata_scm: Fix signature handling of METADATA_REVISION and METADATA_BRANCH

We're not interested in the dependencies of these functions and what
those functions look like, we're interested in the value the variable has.

Force the hashed value to be the actual value from the function. This
means using METADATA_REVISION in DISTRO_VERSION for example now
correctly rebuilds when it changes value.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>