qemu-check.exp: fixes/improvements for Travis CI environment

* Detect kernel rcu_sched stall + cosmetic improvements
* Start QEMU with options '-nographic -monitor none'. Without this,
QEMU cannot be run on the Travis servers which have no display.
'-monitor none' is needed when '-nographic' is set and one wants to
redirect serial port 0 (first occurrence of '-serial stdio'), otherwise
a conflict occurs and the following message is shown:
    qemu-system-arm: -serial stdio: cannot use stdio by multiple character
    devices
* Fix undefined variable error on timeout
* Set LD_LIBRARY_PATH. The libraries that come with the compiler
sometimes end up under /lib, or under /lib/arm-linx-gnueabihf. By setting
LD_LIBRARY_PATH we support both.
* Catch kernel panic/timeout on boot to make the script more robust.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>