phase1: satisfy getver.sh requirements

phase1: satisfy getver.sh requirements

scripts/getver.sh requires upstream branch tracking to correctly compute
revision number. Buildbot Git() step does not set branch tracking as it
checks out individual commits and then reset the target branch to it, so
in order for getver.sh to work, this workaround forcefully sets the
upstream branch.

This fixes malformed version code in filenames as observed on
openwrt-23.05:

r23001+3-38c150612c
instead of:
r23004-7f0db09513

Signed-off-by: Thibaut VARÈNE <hacks@slashdirt.org>