Add replace_grub_root_with_label function

When update-grub is run, it will detect the disks in the build system.
Currently, we sed the grub configuration in places where this happens;
replace_grub_root_with_label moves that seddery in to a single callable
location to avoid ending up with multiple versions of it.

(It's worth noting that we also do this in 999-cpc-fixes.chroot, but as our
functions file isn't available within the chroot we don't use it there.)

live-build/ubuntu-cpc/functions

@@ -249,4 +249,11 @@ convert_to_qcow2() {
     qemu-img info "$destination"
 }
 
+replace_grub_root_with_label() {
+    # When update-grub is run, it will detect the disks in the build system.
+    # Instead, we want grub to use the cloudimg-rootfs labelled disk
+    CHROOT_ROOT="$1"
 
+    sed -i -e "s,root=[^ ]\+,root=LABEL=cloudimg-rootfs," \
+        "$CHROOT_ROOT/boot/grub/grub.cfg"
+}

live-build/ubuntu-cpc/hooks/033-disk-image-uefi.binary

@@ -105,7 +105,7 @@ EOF
 
     chroot mountpoint dpkg-divert --local --rename /etc/grub.d/30_os-prober
     chroot mountpoint update-grub
-    sed -i "s,root=.* ,root=LABEL=cloudimg-rootfs ,g" mountpoint/boot/grub/grub.cfg
+    replace_grub_root_with_label mountpoint
     chroot mountpoint dpkg-divert --remove --local --rename /etc/grub.d/30_os-prober
 
     chroot mountpoint apt-get -y clean

live-build/ubuntu-cpc/hooks/034-disk-image-ppc64el.binary

@@ -52,7 +52,7 @@ EOF
 
     chroot mountpoint dpkg-divert --local --rename /etc/grub.d/30_os-prober
     chroot mountpoint update-grub
-    sed -i "s,root=.* ,root=LABEL=cloudimg-rootfs ,g" mountpoint/boot/grub/grub.cfg
+    replace_grub_root_with_label mountpoint
     chroot mountpoint dpkg-divert --remove --local --rename /etc/grub.d/30_os-prober
 
     umount_partition mountpoint