docs/BLS: clear up the confusion about what $BOOT means

docs/BLS: clear up the confusion about what $BOOT means

The text used was originally written for everything being on the ESP. It was
later generalized for support XBOOTLDR, and "$BOOT" was introduced to mean
something like "XBOOTLDR if present, the ESP otherwise", and most of the text
was changed to talk about $BOOT. Sadly, this doesn't work, because the two
partitions are not interchangeable. sd-boot loads entries from both partitions,
and its configuration, random-seed, etc. only from the ESP.

The terms are redefined: $BOOT now means either the ESP or the "boot partition"
playing the same role on MBR systems, and $XBOOTLDR is XBOOTLDR.

Like various previous commits, this makes the specification describe our
current implementation.

Also, the let's just accept the common practice of using /boot and /boot/efi.
Since both partitions need to be read to gather configuration, it isn't a
problem that one is mounted underneath the other one. I think having /boot and
/efi is OK, but not better in any measureable way, so let's stop trying to push
people towards this setup.

A note that XBOOTLDR must be on the same disk as ESP is added.