kernel.bbclass: move kernel-vmlinux up in PACKAGES

If KERNEL_IMAGETYPE is vmlinux, the expectation is most likely that there will
be no kernel image package, but we still want a vmlinux package for debugging,
so move kernel-vmlinux in front of kernel-image in PACKAGES.

Signed-off-by: Christopher Larson <kergoth@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>