live-server: place kernel in the minimal layer

By placing the kernel in minimal, we can achieve the following
improvements:

1. Space savings - there are redundant packages present in the ship-live
   pool and in the live layer.  Adding the kernel to minimal means that
   the kernel is already in the live layer, and we don't then also need
   it in the pool.
2. Time savings - informal vm testing suggests more than a minute
   improvement to have the kernel preinstalled over installing it at
   runtime.

As always, there is a cost tradeoff:

1. If a different kernel is desired, we need to be able to remove this
   preinstalled kernel.  Relevant curtin and subiquity changes are
   already landed.
2. When installing that other kernel, it'll take longer than today due
   to still needing to install a kernel at runtime + the time cost of
   removing the preinstalled kernel.

live-build/auto/config

@@ -1005,7 +1005,14 @@ case $PROJECT in
 						exit 1
 					fi
 
-					add_package ubuntu-server-minimal.ubuntu-server.installer.$flavor $kernel_metapkg
+					add_pass ubuntu-server-minimal.ubuntu-server.installer.$flavor
+					if [ $flavor == ga ]; then
+						kernel_layer=ubuntu-server-minimal
+					else
+						kernel_layer=ubuntu-server-minimal.ubuntu-server.installer.$flavor
+					fi
+					add_package $kernel_layer $kernel_metapkg
+
 					LIVE_PASSES="${LIVE_PASSES:+$LIVE_PASSES }ubuntu-server-minimal.ubuntu-server.installer.$flavor"
 				done
 				case $ARCH in

live-build/ubuntu-server/hooks/04-kernel-bits.binary

@@ -15,6 +15,9 @@ esac
 
 PROJECT=$PROJECT${SUBARCH:+-$SUBARCH}
 
+# ensure this initrd is casperized
+chroot chroot update-initramfs -u
+
 # Fish out generated kernel image and initrd
 mv chroot/boot/initrd.img-* ${PWD}/livecd.${PROJECT}.initrd-$flavor
 mv chroot/boot/vmlinu?-* ${PWD}/livecd.${PROJECT}.kernel-$flavor