bootctl: replace "type" with "source" in output

bootctl: replace "type" with "source" in output

I think this is more useful (because it's easy to stick the path into an editor command
when one wants to change the options or inspect the files), and more self-explanatory.

Example output:
        title: Fedora 30 (Workstation Edition) (4.20.16-200.fc29.x86_64)
           id: 08a5690a2eed47cf92ac0a5d2e3cf6b0-4.20.16-200.fc29.x86_64
       source: /boot/efi/loader/entries/08a5690a2eed47cf92ac0a5d2e3cf6b0-4.20.16-200.fc29.x86_64.conf
      version: 4.20.16-200.fc29.x86_64
          ...

        title: Fedora 30 (Workstation Edition)
           id: fedora-30
       source: /boot/efi/EFI/Linux/linux-5.0.5-300.fc30.x86_64-08a5690a2eed47cf92ac0a5d2e3cf6b0.efi
        linux: EFI/Linux/linux-5.0.5-300.fc30.x86_64-08a5690a2eed47cf92ac0a5d2e3cf6b0.efi
          ...

        title: Reboot Into Firmware Interface
           id: auto-reboot-to-firmware-setup
       source: /sys/firmware/efi/efivars/LoaderEntries-4a67b082-0a4c-41cf-b6c7-440b29bb8c4f