torture: Choose bzImage location based on architecture

Currently, the scripts hard-code arch/x86/boot/bzImage, which does not
work well for other architectures.  This commit therefore provides a
identify_boot_image function that selects the correct bzImage location
relative to the top of the Linux source tree.  This commit also adds a
--bootimage argument that allows selecting some other file, for example,
"vmlinux".

This change requires that the definition of the QEMU variable be
computed earlier in order to identify where to look for the boot image
when it comes time to copy it to the results directory.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>