Handle the v2019* versioning schema by being less strict

At the moment `ade` enforces a NN.NN versioning schema for sysroot with N
strictly being a digit.

Now that we switched to release versions like v2019dev0, v2019pre, v2019.0 this
is no longer appropriate.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>