waf-samba.bbclass: add missing qemu options

- QEMU_OPTIONS is needed for running binary through qemu,
  add it for CROSS_EXEC.
- "-r ${OLDEST_KERNEL}" is already included in QEMU_OPTIONS,
  remove it from CROSS_EXEC

Signed-off-by: Jackie Huang <jackie.huang@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>