dissect: when mounting an image mount the XBOOTLDR partition to /boot

dissect: when mounting an image mount the XBOOTLDR partition to /boot

Previously, we'd mount the ESP to /efi if that existed and was empty,
falling back to /boot if that existed and was empty.

With this change, the XBOOTLDR partition is mounted to /boot
unconditionally. And the EFI is mounted to /efi if that exists (but it
doesn't have to be empty — after all the name is very indicative of what
this is supposed to be), and to /boot as a fallback but only if it
exists and is empty (we insist on emptiness for that, since it might be
used differently than what we assume).

The net effect is that $BOOT should be reliably found under /boot, and
the ESP is either /efi or /boot.

(Note that this commit only is relevant for nspawn and suchlike, i.e.
the codepaths that mount an image without involving udev during boot.)