build: Stricter git tag version parsing

build: Stricter git tag version parsing

Only consider git tags that begin with 'v', as these are far more likely
to be vpp version tags, and thus parsed by the version script properly.

This needed when working with git clone to which additional tags may
have been added.

eg tags like:

upstream/21.06_rc0+20210622
debian/21.06+20210811-1myvendor2

should be ingored, as they will not be parsed correctly.

Type: make
Change-Id: I7a6d185acd029335abaea5a9dac48839f1397135
Signed-off-by: Nick Brown <nickbroon@gmail.com>