version: Don't force releases to be named in the form of YY.QQ

The current parsing expressions assume release is in the form of
YY.QQ e.g. 16.09. While this is true for Apertis, it's not mandatory for
derivatives so drop that assumption.

While fixing that, move to using match names to make the regexps a bit
more foolproof.

Also split up the function parsing the string from a version file
(/etc/image_version) and from the sysroot version tag as they are
fundamentally different, the current regexp handled both in extremely
subtle and error-prone ways.

Apertis: https://phabricator.apertis.org/T3555



Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Reviewed-by: Frédéric Dalleau <frederic.dalleau@collabora.com>
Differential Revision: https://phabricator.apertis.org/D6264