bootctl: rework Boot Loader Entries section in status

bootctl: rework Boot Loader Entries section in status

Kernels are loaded from two partitions (XBOOTLDR and ESP). In the past this
wasn't always so, but a while ago the docs and the code were updated to load
from both locations. The $BOOT location specifies where to install only. So
rework the status output to print both locations and indicate which one is
$BOOT by appending ", $BOOT".

Also change the section title to "… Locations". We don't show *entries* here,
so the old title could be a bit confusing.

Example:
Boot Loader Entry Locations:
          ESP: /efi (/dev/disk/by-partuuid/31659406-5a17-46ec-8195-0dea1667db58)
     XBOOTLDR: /boot (/dev/disk/by-partuuid/4f8a8fe9-4b45-4070-9e9b-a681be51c902, $BOOT)
        token: fedora