Hooks that only source config/functions (not config/common) would fail
with "LIVECD_ROOTFS_ROOT: parameter not set" when calling setup_mountpoint
or any function that transitively calls it. This affected ubuntu, ubuntu-cpc,
and buildd hooks.
Rather than adding `. config/common` to every hook, prepend the
LIVECD_ROOTFS_ROOT assignment to config/functions at config time so it is
available to all callers automatically.
cbb597a46c