* 03-boot_with_systemd.chroot: Check if /etc/default/grub exists

(temporary fix which to stop build failing - grub will be unconfigured
  though.
* 05-create_minimal_fstab.chroot: Configure a minimal /etc/fstab to
  allow systemd to mount the rootfs read-write.

live-build/ubuntu-core/hooks/03-boot_with_systemd.chroot

@@ -3,7 +3,7 @@
 # Boot using systemd and disable quiet boot
 # to see what is happening.
 systemd=/lib/systemd/systemd
-sed -i \
+[ -f /etc/default/grub ] && sed -i \
     -e "s,^\\([ 	]*GRUB_CMDLINE_LINUX\\)=\"\"$,\\1=\"init=$systemd\",g" \
     -e 's,^\([ 	]*GRUB_CMDLINE_LINUX_DEFAULT\)="quiet splash",\1="",g' \
     /etc/default/grub

live-build/ubuntu-core/hooks/05-create_minimal_fstab.chroot

@@ -0,0 +1,6 @@
+#!/bin/sh -x
+
+cat >>/etc/fstab<<EOT
+# Minimal setup required for systemd to provide a r/w FS
+/dev/root	/	rootfs	defaults	0	0
+EOT