ubuntu-cpc: Ensure base disk image is the same between all build targets

The following targets have livecd.ubuntu-cpc.manifest (and
livecd.ubuntu-cpc.ext4) which differ in some way from the 'all'
target. They are all missing grub-efi and other modifications:

root-dir
squashfs
tarball

These targets do not depend on the 'disk-image' target. This means that
the ext4 produced will lack the uefi modifications (and any from the
disk-image target binary hooks).

Since the ext4 file is common to all builds there is a chance that a
parallel build from one of these targets could overwrite this artifact.
This patch ensures that all targets will produce consistent base output.

live-build/ubuntu-cpc/hooks.d/base/series/root-dir

@@ -1 +1,3 @@
+# Include disk-image to ensure livecd.ubuntu-cpc.ext4 is consistent
+depends disk-image
 base/create-root-dir.binary

live-build/ubuntu-cpc/hooks.d/base/series/squashfs

@@ -1,2 +1,4 @@
+# Include disk-image to ensure livecd.ubuntu-cpc.ext4 is consistent
+depends disk-image
 depends root-dir
 base/root-squashfs.binary

live-build/ubuntu-cpc/hooks.d/base/series/tarball

@@ -1,2 +1,4 @@
+# Include disk-image to ensure livecd.ubuntu-cpc.ext4 is consistent
+depends disk-image
 depends root-dir
 base/root-xz.binary