ubuntu-cpc: in hooks/030-root-tarball.binary create /lib/modules to fix

(LP: 1543204).

This allows kernel + initramfs + cloud-initramfs-copymods to use the lxd
image.  Without /lib/modules present the cloud-initramfs-copymods
initramfs module will not be able to populate /lib/modules/<uname -r>.

debian/changelog

@@ -24,6 +24,8 @@ livecd-rootfs (2.376ubuntu1) UNRELEASED; urgency=medium
   [ Ben Howard ]
   * ubuntu-cpc: fix hooks/032-disk-image.binary call to
     create_empty_partition, which requires five args due to "-u"
+  * ubuntu-cpc: in hooks/030-root-tarball.binary create /lib/modules to fix
+    (LP: 1543204).
 
   [ Dimitri John Ledkov ]
   * Do not remove linux-base, when purging all the linux-*, in the tarball

live-build/ubuntu-cpc/hooks/030-root-tarball.binary

@@ -22,6 +22,7 @@ chroot binary/boot/filesystem.dir chmod +x /usr/sbin/grub-probe
 env DEBIAN_FRONTEND=noninteractive chroot binary/boot/filesystem.dir apt-get --purge remove --assume-yes '^linux-.*' 'linux-base+'
 env DEBIAN_FRONTEND=noninteractive chroot binary/boot/filesystem.dir apt-get --purge remove --assume-yes '^grub-.*'
 env DEBIAN_FRONTEND=noninteractive chroot binary/boot/filesystem.dir apt-get autoremove --purge --assume-yes
+chroot binary/boot/filesystem.dir mkdir /lib/modules
 
 chroot binary/boot/filesystem.dir rm /usr/sbin/grub-probe
 chroot binary/boot/filesystem.dir dpkg-divert --remove --local --rename /usr/sbin/grub-probe