acme-common: Create challenge directory on boot

acme-common: Create challenge directory on boot

The challenge directory (for webroot challenges) is on a tmpfs, which
means it doesn't exist on boot. Some web servers (uhttpd in particular)
don't like being configured to serve files from a non-existent
directory. So add a boot() section to the ACME init script that just
creates the challenge directory, and make sure it runs relatively early.
That should take care of the non-existent directory issue, while still
keeping the actual certificate renewal controlled by cron.

Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>