sstate.bbclass: allow to disable sstate package creation

* building core-image-minimal-mtdutils with and without sstate packages enabled:
          with   without
  real     98m       95m
  user    212m      208m
  sys      26m       25m

* but this was tested on fast machine with TMPDIR on fast raid0 array
  so the impact is not so bad on simple image, but stuff like qt4 takes
  ages on our slow buildhost

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
qemu.inc: remove EXTRA_IMAGEDEPENDS, images are usefull even without qemu installed on host

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>