kernel: make kernel-base recommend kernel-image, not depend

As kernel modules depend on the base kernel package, and the base kernel
recipe depends on the kernel image, it's impossible to build file system
images which contain kernel modules but not the kernel itself, such as
an initramfs.

Change the RDEPENDS to RRECOMMENDS so that the disk images can set
PACKAGE_EXCLUDE = "kernel-image-*" to remove the kernel.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>