Do not use tailf to wait for qemu

The linux-runner hangs on travis, instead of using tailf to wait for
qemu, we grep the log directly. This stopped the hang.

docker/linux-runner

@@ -70,13 +70,29 @@ esac
  fi
 
  # wait for dropbear
- tailf $LOG | grep -m1 "Not backgrounding" > /dev/null
+ for i in $(seq 240); do
+ if grep -q "Not backgrounding" $LOG; then
+ READY=1
+ break
+ fi
+ sleep 0.5s
+ done
+
+ if [ -z "$READY" ]; then
+ if [ -n "$CROSS_DEBUG" ]; then
+ echo "Not ready but continuing because CROSS_DEBUG is set"
+ else
+ echo "Qemu is not ready..."
+ echo "Set the environment variable CROSS_DEBUG=1 to debug"
+ exit 1
+ fi
+ fi
 
  echo Booted in $(dbclient -K 1 -p 10022 -y -y root@localhost "cut -d' ' -f1 /proc/uptime") seconds
 
 ) 200>$LOCK
 
-exec dbclient \
+dbclient \
  -t \
  -p 10022 \
  -y -y \