qemuboot: write the full kernel filename, not the link name
KERNEL_IMAGETYPE gives the filename of a symlink to the kernel,
which may not be available i.e. if the user downloads some build
artefacts to run on a local machine. It's also possible that the
link will point to a newer kernel than was intended for use with
the rootfs in the qemuboot.conf.
It's much more reliable to read the name of the file
KERNEL_IMAGETYPE is linking to and assign the full filename to
QB_DEFAULT_KERNEL.
[YOCTO #10285]
Signed-off-by: Joshua Lock <joshua.g.lock@intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
