kernel.bbclass: Avoid wildcards for kernel images

kernel.bbclass: Avoid wildcards for kernel images

With multiple kernel images enabled starting with 'vmlinux', e.g.
vmlinux.gz and vmlinux.bin, all files landed inside the
kernel-vmlinux package.

On top of that, even initramfs images were included, e.g.
vmlinux.gz-initramfs-*.

Signed-off-by: Andreas Oberritter <obi@opendreambox.org>