qemu-check.exp: fix expected prompts

Since commit eed314d1 ("qemu: build using buildroot"), the login
and the shell prompts of the root FS have changed. Adjust our "make
check" accordingly.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>