And enable it by default, since it's a sensible thing to do.

This makes the bootmisc config file unnecessary, since it was only
used before to make sure that /tmp was wiped on every boot.

Mounting /tmp as a tmpfs will be skipped if:
* The user or maintainer configured deviceinfo_tmp_as_tmpfs_size=0
* If they didn't but the device has less than 2GB of RAM
* And in any case, if it is already mounted, to respect users that
  might have it in /etc/fstab

The options for mounting /tmp has been copied from my local debian
tmp.mount service. The only real difference is that we are mounting it
after /etc/fstab, and they do so before.

Fixes #2233