testlab.bbclass: use opkg.conf from staging, always specify tmp_dir in opkg-cl call...

testlab.bbclass: use opkg.conf from staging, always specify tmp_dir in opkg-cl call with -t parameter

* Use opkg.conf from staging in the same way as do_rootfs does
* When option tmp_dir is used in opkg.conf installed on rootfs then it's
  used also in do_rootfs call and points to probably non-existent directory
  on buildhost like /var/lib/opkg/tmp.
* The value of tmp_dir from rootfs is used even with another config file
  specified with -c parameter
* Before this, it was using default value (/tmp) on buildhost, now it will use
  own "${IMAGE_ROOTFS}-tmp" and remove it after do_rootfs finish
  (usually already empty inside, cleaned by opkg itself)

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Acked-by: Denys Dmytriyenko <denis@denix.org>
Acked-by: Koen Kooi <koen@openembedded.org>