scripts: dtc-version: support git version strings too

Building dtc from git causes the version number to start with a 'v'
(e.g. v1.7.0). printf then fails to parse 'v1' as a decimal value, and
prints '000700' instead of '010700'. Subsequently, the build fails,
because '000700' is less than the required '010400' version.

Signed-off-by: Martin Hundebøll <martin@geanix.com>