terminal.bbclass: Use BB_ORIGENV to restore user environmental variables

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
imagetest-qemu: Adapt to bitbake environment changes

Load environment variables like DISPLAY from BB_ORIGENV after recent
bitbake changes.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>