core-image-minimal-initramfs: don't install a kernel into the initramfs

core-image-minimal-initramfs: don't install a kernel into the initramfs

There's obviously no point in installing a kernel into an initramfs, but
if we install kernel modules then they'll recommend the relevant kernel.

Set PACKAGE_EXCLUDE to ensure the initramfs for a kernel doesn't contain
the kernel.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>