cmake: qemu: Restore QEMU_PTY/QEMU_PIPE option handling.

Both options were originally parameters to *make*, not cmake, so people
can augment QEMU ibocation from run to run, not from complete rebuild
to complete rebuild. Make them such again.

However, just in case, ability to set "default" value for those options
on CMake level is preserved.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>