kernel.bbclass: Use real filenames in kernel packages

kernel.bbclass: Use real filenames in kernel packages

When iterating over kernel image types to set up their packaging
variables, don't use make targets but the real names.

It was surprising if both vmlinux.bin and vmlinux.gz were enabled
and only the latter had its filename extension removed from the
package name.

Signed-off-by: Andreas Oberritter <obi@opendreambox.org>
Signed-off-by: Ross Burton <ross.burton@intel.com>