Possible bug in configuration-system?

[Raghav Gururajan <rg@raghavgururajan.name>]

Hello Guix!

As mentioned in this page of manual 
(http://guix.gnu.org/manual/en/html_node/Mapped-Devices.html), for 
lvm-device-mapping, I did the following in my config.scm:

```
  (mapped-devices
   (list
    (mapped-device
     (source
      (uuid "47bb32fe-da7f-4eb6-a11d-ae07a3a2e6f8"))
     (target "secondary")
     (type luks-device-mapping))
    (mapped-device
     (source "secondary")
     (target (list "secondary-root" "secondary-home"))
     (type lvm-device-mapping))))
```

When I did `guix system reconfigure`, I got the following error:

```
Backtrace:
In ice-9/boot-9.scm:
   1736:10 19 (with-exception-handler _ _ #:unwind? _ # _)
   1731:15 18 (with-exception-handler #<procedure 7fe09e8b7930 at ic…> …)
In guix/scripts/system.scm:
    1391:8 17 (_)
In guix/status.scm:
     780:4 16 (call-with-status-report _ _)
In guix/scripts/system.scm:
    1219:4 15 (_)
In ice-9/boot-9.scm:
   1736:10 14 (with-exception-handler _ _ #:unwind? _ # _)
In guix/store.scm:
    636:37 13 (thunk)
    1305:8 12 (call-with-build-handler #<procedure 7fe09e672720 at g…> …)
   2062:24 11 (run-with-store #<store-connection 256.99 7fe09786ab40> …)
In guix/scripts/system.scm:
   1243:15 10 (_ _)
     813:9  9 (perform-action reconfigure #<<operating-system> kerne…> …)
In gnu/system.scm:
   1312:26  8 (operating-system-bootcfg #<<operating-system> kernel:…> …)
    574:14  7 (operating-system-bootloader-crypto-devices #<<operatin…>)
In unknown file:
            6 (filter #<procedure 7fe09580d7a0 at gnu/system.scm:551…> …)
In gnu/system.scm:
    552:26  5 (_ _)
    528:17  4 (mapped-device-users #<<mapped-device> source: "second…> …)
In srfi/srfi-1.scm:
    586:17  3 (map1 (("secondary-root" "secondary-home")))
In unknown file:
            2 (string-append "/dev/mapper/" ("secondary-root" "seco…"))
In ice-9/boot-9.scm:
   1669:16  1 (raise-exception _ #:continuable? _)
   1669:16  0 (raise-exception _ #:continuable? _)

ice-9/boot-9.scm:1669:16: In procedure raise-exception:
In procedure string-append: Wrong type (expecting string): 
("secondary-root" "secondary-home")
```

When I do the following in my config.scm, system reconfigure works fine:

```
  (mapped-devices
   (list
    (mapped-device
     (source
      (uuid "47bb32fe-da7f-4eb6-a11d-ae07a3a2e6f8"))
     (target "secondary")
     (type luks-device-mapping))
    (mapped-device
     (source "secondary")
     (target "secondary-root")
     (type lvm-device-mapping))))
```

So seems like listing more than one target causes the error. Is this a bug?

Regards,
RG.