package/18xx-ti-utils: use short version string

This is compatible with release-monitoring.org, so it allows easier
tracking of update status.

There appears to be no easy way to use anything other than tag/commit
for _VERSION with the git download method. git.ti.com offers snapshot
download over https, so use use that instead. As added bonus, the https
download method is more firewall friendly.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>