jenkins-build-common: use 'git submodule update'
Now that we use git submodules in osmo-trx, make sure to initialize and
update them before attempting to build.
Fix for:
Makefile.am:32: error: required directory ./osmocom-bb/src/host/trxcon does not exist
Change-Id:
Ic2b9207b942a8a9edff82737117b2ed9d6d3cfe3