jenkins_common.sh: fix build_bts distcheck for more than one conf_flag
jenkins_common.sh: fix build_bts distcheck for more than one conf_flag

Passing configure flags in DISTCHECK_CONFIGURE_FLAGS requires enclosing all
flags in quotes. Currently we seem to have no callers with more than one
configure flag, so we were lucky not to break there.

Change-Id: I37bc517a30d00c744eddc8565a0a8181cb3b2cdb

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts