build: Restrict tag names when discovering version

build: Restrict tag names when discovering version

Type: improvement

When `make install-ext-deps` is called, it creates a debian changelog
file and use `git describe` to fill project version.  On downstream
consumers it's possible to have different tags that makes it to end up
using an invalid version on changelog and breaking the installation
process.

Restrict tags to be considered by git-describe to match 'v[0-9]*'
pattern to get it fixed.

Signed-off-by: Renato Botelho do Couto <renato@netgate.com>
Change-Id: Ieabd7b42ac33735ec4d484bed9039ff20c9872f2